9834
Ind. Eng. Chem. Res. 2008, 47, 9834–9841
APPLIED CHEMISTRY Dissolution of Mineral Fiber in a Formic Acid Solution: Kinetics, Modeling, and Gelation of the Resulting Sol Henrik Gre´nman, Fernando Ramirez, Kari Era¨nen, Johan Wa¨rnå, Tapio Salmi, and Dmitry Yu. Murzin* Laboratory of Industrial Chemistry and Reaction Engineering, Process Chemistry Centre, Graduate School of Materials Research, Åbo Akademi UniVersity, Biskopsgatan 8, FI-20500 Åbo/Turku, Finland
A new inorganic adhesive based on sol-gel technology has been developed recently. The sol is produced by dissolving a mineral material, with silica as the major compound, in formic acid. In this work, the kinetics and mechanisms of the solid-liquid reaction of the amorphous mineral raw material with formic acid were studied. The effect of different variables, such as temperature, acid concentration, and fiber load, were examined. Moreover, the influence of various parameters on the gelation rate of the resulting sol was investigated at different temperatures. The dissolution of the mineral fiber was found to be strongly dependent on the temperature and fiber load, but moderately dependent on the acid concentration as the pH of the solution was buffered by the formic acid. The kinetic model developed for the dissolution process accurately describes the experimental results and can thus be used for design and optimization of the process. The gelation studies showed that the temperature and the amount dissolved are the variables most affecting the gelation time. The knowledge of the gelation kinetics is important when striving for a high dry solid content in the sol, while still avoiding premature gelation in production, transport, and storage. 1. Introduction In the quest for reducing energy consumption in the world, insulation and the materials used for it play a crucial role. The European Insulation Manufacturers Association (EUMIRA) reported in 1990 that “310 million tonnes of heating-related emissions can be avoided every year by applying state of the art thermal insulation measures to new and existing buildingss some 50% of the total heating-related emissions and well over 10% of the total CO2 emissions.”1 Building insulation has been considerably improved since 1990, especially after the application of the new EU directives, resulting in notable energy savings. Nevertheless, it is still possible to reduce the CO2 emissions by 83 million metric tons/ year by 2010, rising to 144 million metric tons/year by 2015, providing cost and energy savings of 8 billion euros/year by 2010 and rising to 14.5 billion euros/year by 2015.1 Application of organic compounds, such as polyurethane foams, nitrogen-based urea-formaldehyde (UF) foam, phenolic foam, etc., as thermal insulators has been widely investigated previously.2,3 Inorganic insulating materials have been studied, too. The most traditional one used in building insulation is fiberglass made from molten glass spun into microfibers. Comparisons made between different insulating materials showed that polyurethane and polystyrene compounds have the lowest thermal conductivity, followed by fiberglass and rock/ mineral wool.4 Nevertheless, polyurethane and polystyrene are combustible materials requiring the use of fire retardants5 (traditionally halogen or phosphate-containing compounds). These compounds are environmentally dangerous, and therefore, the use of inorganic insulating materials is more desired. Mineral fiber has shown good properties as a substitute for organic compounds commonly employed in construction. * To whom correspondence should be addressed. Tel.: +358-2-215 4985. E-mail:
[email protected].
Nevertheless, even though the fibrous material is inorganic, the adhesive keeping them together is typically an organic product6,7 such as phenol resins. The binder is neither as fire retardant nor environmentally friendly as desired, and it contributes significantly to the total cost of the insulation material. Recently, a new method for the preparation of a colloidal silicate dispersion was introduced.8 According to the results, an economically feasible silicate binder with fire resistant and good binding properties, was prepared from a colloidal dispersion with a low content of alkali oxides. The dispersion could, in particular, be easily transformed into a gel by changing the pH. Acetic acid concentrations over 1.5 M increased the stability of the resulting sol, hence increasing the gelation time. Nilsen et al.9 developed a new low-cost inorganic binder for use in the production of fiber insulation, using an amorphous mineral raw material containing silica as the major component. The sol was prepared by dissolving the precursor in formic acid. The effect of the acid concentration on the dissolution rate was studied in a limited number of experiments, with 4.6 wt % mineral material, and it was concluded that pH and the dissolution time of the fiber depend on the concentration of the solution. The stability of the obtained silica sol was increased by Puputti et al.10 through introducing ethanol in the aqueous solution. An optimum of around 35% ethanol in the initial solution was observed to result in a significant increase of the gelation time. No significant influence of ethanol on the dissolution kinetics was observed. In 2005, Nilsen et al. published a paper11 in which the composition of the mineral fiber was determined. The effect of the acid concentration on the dissolution kinetics and the gelation was studied. Acid concentrations exceeding 2 M did not show any significant difference in the dissolution rate for the different fiber loads tested. However, an optimum between 2 and 6 M acid
10.1021/ie800267a CCC: $40.75 2008 American Chemical Society Published on Web 11/20/2008
Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 9835
concentration was found to maximize the stability of the sol, leading to higher gelation times. None of the research mentioned above has studied the dissolution kinetics deeply. In addition, the mineral fiber dissolution mechanism remains unclear. The factors affecting the kinetics are mass and heat transfer in the reactor, fiber load, acid concentration,12 dissolution temperature, and dissolution time as well as the morphology and surface area of the mineral precursor. The goal of the present research is to investigate the dissolution of the low-cost amorphous mineral raw material in formic acid, as well as to derive a kinetic model for the heterogeneous solid-liquid reaction. Moreover, the influence of the dissolution and gelation parameters on the gelation time has been studied in the present work. A kinetic model describing the dissolution is essential for dimensioning reactors in order to obtain the desired capacity in production. It is also an important tool in optimizing reaction parameters such as temperature, concentrations, and residence time for achieving a well-controlled production with the exothermic reaction as well as achieving good product quality and optimizing the economics of production, e.g., the use of acid. Moreover, the knowledge of gelation kinetics is crucial when striving for a high dry solid content in the sol, but avoiding premature gelation during production, transportation, and storage. 2. Experimental Procedures 2.1. Equipment. Dissolution with lower fiber loads was initially performed under nitrogen flow in a 1 L isothermal glass reactor containing 750 mL of liquid. From the viewpoint of industrial implementation it is obvious that application of an inert gas imposes a serious hurdle; therefore the influence of oxygen was specifically elucidated and it was concluded that this influence could be neglected. Thus, the following experiments allowing also increased fiber load were performed in an open 2 L decanter glass containing 1 L of liquid, which was placed on a heating plate and equipped with an internal cooling coil, for practical ease. The reactor systems included deflecting baffles to ensure turbulent flow. Online conductivity and temperature measurement sensors were attached to the baffles. The temperature control was concluded to be accurate in both reactors. Mixing was performed by a two-propeller stirrer with a diameter of 6.5 cm and consisting of eight blades. Phase separation of the remaining solid and liquid was performed by either filtration or centrifugation, depending on the experimental conditions. A Rheomat viscosimeter was used for determining the point of gelation. Inductively coupled plasma mass spectrometry (ICP-MS) was applied for determining the concentration of ions in the liquid phase during dissolution. 2.2. Materials and Dissolution Procedure. The mineral fiber precursor is a mixture of different inorganic compounds such as SiO2, Al2O3, CaO, MgO, and FeO. It can be viewed as a tangle of short filaments held loosely together by a binder (see Figure 1), which has the same chemical composition as the fiber, making sufficient mixing challenging. Formic acid, diluted with distilled water, was used as the solvent. It should be noted that formic acid acts as a buffer in the pH range 2.75-4.75. The fiber load was varied between 30 and 113.33 g/L, and the concentration of formic acid was varied between 1.2 and 9.5 mol/L. The liquid reactant was in excess in all the experiments. An average molar weight for the fiber was calculated, based on the ICP-MS results, giving a value of 60.25 g/mol. Hence the used fiber loads can also be expressed as being varied between 0.5 and 1.88 mol/L. The studied temperature
Figure 1. SEM image of fresh mineral fibers. The scale in the upper corner indicates 50 µm.
range was 13-50 °C, and the reaction time was varied between 10 and 180 min. To ensure efficient mixing during the reaction, the mineral fiber was subjected to a preceding wetting process with sufficient stirring, until the solid particles were well dispersed. After the desired temperature was reached, the reaction was started by introducing the required amount of formic acid into the reactor. To investigate the influence of external mass transfer, the mixing speed was varied between 100 and 400 rpm. It was concluded that with agitation rates above 220 rpm no differences in the dissolution rate were observed and intrinsic conditions were achieved. Therefore as the standard mixing speed 400 rpm was used in the experiments, allowing good dispersion of the solid phase. The presence of two different solid phases was observed after the reaction, when the mixing was stopped: the unreacted fiber, which rapidly settled to the bottom of the reactor, and a much lighter solid phase which remained in the suspension and caused problems in filtering. The separation problem was solved by using a large centrifuge. The efficiency of the centrifugation was verified by filtering the sol, and it was concluded to be sufficient. The wet solids in the bottles were dried at 100 °C for over 12 h and weighed, to determine the amount of undissolved fiber. In a number of experiments, a small amount of the solution, which was not centrifuged, was filtered and around 50 mL of sol was obtained. The filtered liquid was used for the gelation experiments. The final pH of the solution was measured in a number of experiments. 2.3. Gelation Experiments. Filtered liquid from the dissolution experiments was utilized for measuring the gelation time of the sol. The viscosimeter measurement system, made of stainless steel, consisted of a measuring bob placed in a tube with a closing cap. The tube was placed inside a water bath with temperature control. Gelation experiments were performed at 16, 20, 30, and 40 °C. Measuring was conducted with a shear rate of 600 s-1 and a viscosity measurement range starting from 3 mPa · s. Figure 2 shows the evolution of viscosity with time in a gelation experiment where 76 g/L fiber is dissolved, performed at 40 °C. A sudden increase in the viscosity was noticed (after 200 min in this case), i.e., gelation. An average value of 10 mPa · s is considered as the point of gelation. This is the point where the viscosity shows an abrupt increase, and where gelation can no longer be interrupted or prevented. This
9836 Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 Table 1. Mineral Fiber Composition, with Major Compounds (Accounting for >98 wt %) Calculated as Oxides and Compared to Literature Data element ICP analysis, mg/L compound solid fiber, % Nilsen et al.11
Figure 2. Example of the evolution of viscosity during a gelation experiment. The experiment was performed at 40 °C with 76 g of fiber dissolved per liter of solvent.
Figure 3. ICP analysis of the main compounds in the liquid phase. The reaction was performed with 113 g/L solid and 4 M formic acid at 13 °C.
definition of the gelation point is supported by other studies employing the same criteria to define the point of gelation.13 2.4. ICP-MS and Particle Size Measurement. Inductively coupled plasma mass spectrometry (ICP-MS) was used for determining the concentration of ions in the liquid phase during the dissolution. The ICP analysis was performed in an experiment at 13 °C, which was the lowest temperature feasible in practice, in order to retard the reaction rate facilitating kinetic observations. For the analysis, seven samples of the sol were withdrawn at different stages of the reaction. Laser diffraction was used to study how the radial particle size distribution changes during the reaction. The lens was submerged into the reaction mixture, and the particle size distribution was measured with intervals of a few minutes during the whole experiment. This was repeated in a number of experiments with different reaction conditions. 3. Experimental Results 3.1. Dissolution Experiments. 3.1.1. ICP-MS Analysis. The analyzed compounds (Al, Ca, Fe, Mg, Si, Cr, Mn, Ti, K, Zn, Ni, Sr) showed concentrations in the range 0.5-18 000 mg/ L. However, the analysis of the most significant cations Si, Al, Ca, Mg, and Fe (representing over 98% of the fiber as oxides) showed concentrations in the range 800-18 000 mg/L. Figure 3 illustrates the concentrations of the main compounds in the liquid phase, i.e., the sol, as a function of time. The ratio of the elements in the liquid phase remained constant after 20 min. Therefore, the composition of the mineral fiber precursor can be calculated from the concentration of the different cations in the solution. The composition of the solid fiber is depicted in Table 1 and compared with the previous work, which has utilized the same mineral precursor.11 As can be seen, the values are in good agreement with previous research.10,11 Based on
Si Ca Al Mg Fe K Ti Sr Mn Cr Ni Zn
16050 9157 6584 5650 3812 538 479 113 70 27 4 2
SiO2 CaO Al2O3 MgO FeO
46.5 17.3 16.8 12.7 6.6
42 30 17 6
Figure 4. Conductivity as a function of amount dissolved at 25 °C. The initial concentrations of solid and formic acid were 0.76 mol/L and 2.5 M, respectively.
Figure 5. Change in conductivity (0) and amount of fiber dissolved ([) as a function of time. The reaction was performed at 20 °C with a fiber load of 113 g/L and a formic acid concentration of 4 M.
the composition of the material, a mean value for the molar mass of the mineral fiber can be calculated, giving 60.25 g/mol. 3.1.2. Dependence of Conductivity and pH on the Amount Dissolved. During the dissolution experiments, online measurements of conductivity and temperature were applied. A relation between the conductivity and the amount of fiber dissolved during the reaction was expected14 based on the increase of the concentration of ions in the liquid phase during the dissolution. The relation between the amount dissolved and conductivity as well as pH was studied. In cases of low fiber loads and lower acid concentrations, a linear correlation between the conductivity and the amount of fiber dissolved was observed, which is displayed in Figure 4. However, this simple linear relation is no longer valid in cases of higher fiber loads and acid concentrations. Figure 5 shows the evolution of conductivity during time for an experiment where a higher fiber load and acid concentration was used. The shape of the pH curve is almost identical to that of conductivity, which indicates that conductivity is strongly influenced by the concentration of the hydronium ion. It can be noticed from Figure 5 that the conductivity does not correspond directly to the amount dissolved. The conductiv-
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Figure 6. Effect of temperature on dissolution with 4 M formic acid concentration and 113.33 g/L fiber. The solid lines represent the fit of the mechanistic model (see 4.2. Kinetic Model) to the experimental data.
Figure 8. Microscopic image of partly dissolved fibers.
Figure 7. Effect of formic acid concentration on dissolution with an initial solid load of 113 g/L. Dissolution time was 30 min at 20 °C and 20 min at 40 °C.
ity profile (and by extension the pH) shows a behavior typically corresponding to some kind of saturation, which in this case seems to be reached after about 20 min of reaction, while the amount of dissolved fiber was still increasing during the 60 min. For this reason, neither conductivity nor pH can be used to predict the amount of dissolved fiber except for in the very early stages of the reaction. A similar behavior of pH has been observed in a previous study.8 3.1.3. Effect of Temperature on the Dissolution Rate. The influence of temperature on the kinetics was studied in numerous experiments with different solid loads and acid concentrations. A significant number (32 of 73 in total) of the experiments were performed in a 4 M formic acid solution with a solid load of 113.33 g /L. The experiments revealed that the dissolution rate strongly depends on the reaction temperature. Close to the maximum conversion, the concentration was so high that the sol started gelating already inside the reactor at the highest temperatures. The influence of temperature on the reaction rate can be clearly seen in Figure 6 (note that each point corresponds to a separate experiment). The experiments performed at 13 °C show an S-shaped curve, present also in the experiments performed at 20 °C, more visible in Figure 5. The dissolution rate seems to be lower in the first minutes, reaching a maximum after ca. 20 min of reaction in the case of dissolution at 13 and 20 °C. The S-shaped kinetics could be explained by an increase in the surface area of the particles during the early stages of the reaction. Nitrogen adsorption experiments suggested a significant increase in the surface area. Fresh samples exhibited values in the range 1-2 m2/g, while the partially dissolved samples gave values between 3 and 5 m2/g. The increase in the surface area is also supported by an observation in Figure 8. As the method and equipment used are not specifically designed for solid materials exhibiting such low values of surface area (
