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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 mineral bentonites by hydrochloric or sulfuric acid, considerable 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 ammonium 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 ammonium 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 refreshing the recycled dilute acidic solution. * Corresponding author. E-mail:
[email protected]. Fax:+36 13257554.
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 performance of the activated bentonite is illustrated by its bleaching capacity, which also was investigated as a function of the recycling steps, whereas characterizations via surface acidity, IR spectra, and surface area will be addressed in a following paper. Experimental Section Materials and Methods. The bentonite was obtained 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 (commercial 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 spectrometer (Thermo Jarrel Ash). In the case of silicon, a gravimetric method was applied. UV-vis measurements were performed with a Unicam UV4 UV/VIS spectrophotometer. 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 cm3 of concentrated HCl was added, and the resulting mixture was boiled for 5 min.
10.1021/ie990650f CCC: $19.00 © 2000 American Chemical Society Published on Web 09/14/2000
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After filtration, the residual SiO2 was calcinated and weighted. The metal content, with the exception of sodium, was measured by ICP spectrometry. (b) For sodium measurement, 0.25 g of the powdered and homogenized bentonite sample was wetted with a few drops of double distilled water, and after the addition of 10 cm3 of concentrated HF, the sample was evaporated to dryness. This treatment was repeated, then 2 × 10 drops of concentrated sulfuric acid were added, and the mixture was dissolved in doubly distilled water. The sodium concentration of the solution prepared in this way was measured by ICP spectrometry. Acid Treatment of Bentonite. Powdered, air-dried bentonite (4.5 g) was refluxed with a mixture of 400 cm3 of concentrated nitric acid and 600 cm3 of water for 1.5 h in a round-bottom flask. After the removal of the acidic solution via decantation, the bentonite was washed with doubly distilled water until it had become acid-free, and then it was dried at room temperature. In the recyclization experiments, the samples were treated in the same manner with an activating solution refreshed by concentrated nitric acid. Measurement of the Bleaching Force of the Activated Bentonite. Technical-grade vaseline (light brown, semisolid) was treated with 0.6 g of activated bentonite on a water bath for 15 min; then the liquid material was filtered. After a repeated treatment, a white solid vaseline was obtained. The purity of the vaseline was determined by UV-vis spectrometry in CCl4 solution. Pharmaceutical-grade vaseline was used for the preparation of a blank. Aromatics in the UV range and color components in the visible range can both be detected. Results and Discussion Montmorillonite is the active component of the bentonite used for the manufacture of bleaching earth. The montmorillonite is a three-layer phyllosilicate. The base structure is Al4Si8O20(OH)4, but natural montmorillonite always contains some substituents in the lattice. The structure is composed of two sheets of silicate tetrahedra (Si can be partly substituted by Al) and one of alumina octahedra (Al can be partly substituted by Mg2+, Fe2+, or Fe3+). According to Mo¨ssbauer spectra,11,12 the valence state of Fe in the montmorillonite is typically 0-8% ferrous and 92-100% ferric ion. An important consequence of the substitution of Al by Mg or Fe2+ and Si by Al is the generation of negative charges on the layers. Because of charge neutrality, positive cations such as Ca2+ and Na+, and to a lesser extent K+, Mg2+, Fe2+, Fe3+, and Al3+, are built into the crystal lattice. The first step of the acid activation process is the ion exchange of the outer-sphere cations by hydrogen ions and the formation of H-bentonites.13 Under more vigorous conditions (e.g., in the present case), the nitric acid attacks the crystal lattice and Al, Fe, and Mg are released. There are two explanations for the mechanism of this process. According to the first, intramolecular auto-exchange between the aluminum in the lattice and the hydrogen ions on the surface produces Al-exchanged montmorillonite14,15 at reflux temperature. The outer Al ions can then reacts with the nitric acid, thus forming H centers again. The other process is a direct attack of the acid on the crystal lattice. After a critical amount of lattice ion is released, the crystal lattice is broken, and bleaching earth with high surface area is formed. In this case, the residual metal content of the bleaching
earth is small. After further acid treatment (for long reaction times), the residual cations are also released from the broken crystal lattice,16,17 the bleaching earth becomes passivated, and silica is formed.5 Surface acidity is one of the important parameters characterizing bleaching properties.2,15-18 The aluminum ions on the surface of the activated bentonite are the sources of Brønsted or Lewis acidic sites responsible for surface acidity; thus, aluminum resorption is advantageous.15,16 The cations released from the crystal lattice during nitric acid treatment at about 100 °C (at the given acid concentration and reaction time) were Al3+, Fe3+, Mg2+, Ca2+, Na+, and K+. The waste nitric acid solution contains unreacted nitric acid, the nitrate salts of the mentioned major elements, and the nitrate salts of trace elements also leached from the bentonite. When the waste acid reacts with ammonia, the nitric acid forms ammonium nitrate and the hydrolizable cations (Al, Fe, and Mg) produce the appropriate metal hydroxides and ammonium nitrate, whereas the nitrate salts of nonhydrolizable cations (Ca, Na, or K) do not react with ammonia. In this way, the nitrate salts of hydrolizable cations are nitric acid precursors in ammonium nitrate production
M(NO3)x + xNH3 + xH2O ) M(OH)x + xNH4NO3 where M ) Al, Fe, or Mg; and x ) 2 or 3. The valence state of the iron in the nitric acid solution is +3, as iron(II) is oxidized by air and nitric acid during acid treatment. As far as NH4NO3 production is concerned, it can be concluded that the amount of HNO3 lost is equal to the amount (in units of moles) of nonhydrolizable metal nitrate content. However, this nitrate content also acts as a nitrate source in the complete fertilizer product. The ammonium nitrate solution/suspension containing the metal hydroxide ballast materials can be transported by slurry pumps to the fertilizer granulating cylinder. The results of composition measurements of the initial bentonite and the acid-activated bentonite are shown in Table 1. From these data, we calculated the nitric acid distribution between the free acid and the acid content of nonhydrolizable and hydrolizable metal nitrate forms. In these experiments, ca. 36.3% of the mass of the bentonite was dissolved in the nitric acid. The distribution of the nitrate in the waste acid was as follows: free acid, 91.4%; acid in hydrolizable metals salts, 8.2%; and acid in nonhydrolizable metal salts, 0.4%. Naturally, the exact amounts of the released metal ions depend on the initial acid concentration, the reaction time, the composition, and the amount of dissolved bentonite. Thus, from the viewpoint of ammonium nitrate production, the loss of nitric acid is only 0.4% at a 36.7% weight loss of the bentonite. One part of the metals in the outer sphere is nonhydrolizable, and the other part is hydrolizable. In general (for the outer-sphere cations), the amount of hydrolizable cations exceeds the total sum of nonhydrolizable cations; therefore, the loss of nitric acid is lower than the total nitric acid equivalence related to the outer-sphere ions. During further activation, the amount of hydrolizable metals released from the crystal lattice increases. The loss of nitric acid can also be attributed to the nonhydrolizable metal content of accompanying minerals.
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The expression ξi,j contains the measured metal concentrations in the initial and activated bentonite samples. The physical-chemical meaning of this coefficient is the amount of dissolved metals from the initial bentonite samples, in kilograms of metal per kilogram of bentonite. Because the amount of silicon is constant during the entire procedure (not dissolved in HNO3), v1 and f1 can be calculated from the change in the silicon concentration.
Table 1. Composition of the Raw Bentonite and the Activated Bleaching Eartha activated bleaching earth
bentonite SiO2 TiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O Ba Co Cr Li Mn Ni P Pb V Zn
Main Components, % (w/w) 46.72 1.86 15.91 9.75 2. 33 1.11 0.18 0.70
73.29 2.31 5.40 1.36 0.19 0.12 0.18 0.55
AcSi ) f1Ac*Si
After the first cycle, a new charge of bentonite was added to the recycled acidic solution, and the amount of the acid consumed and its concentration were restored by the addition of nitric acid. In this way, the initial acid/bentonite ratio could be kept at a constant value. Consequently, both the amount of nitric acid (67%) and the amount of bentonite used change as a function of cycle number. Thus, in each cycle, both the dissolution degree of the bentonite and the degree of metal ion release will be the same.
Trace Elements (
