Ferrosilicon - reaction mass of iron and iron disilicide and iron silicide and silicon

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The vapour pressure of FeSi (all) grades is extremely low and the substances and constituents are in practice in-volatile substances (at NTP). However, FeSi may be emitted from anthropogenic sources into the atmosphere in gaseous form or as small particles which eventually form aerosols in the atmosphere. Aerosols are then being washed out after a certain residence time.

FeSi in particulate form is immobile in soil and sediment. Dissolution rate of different FeSi alloys is generally low, but dependent also on grade and environmental conditions. Adsorption/desorption behaviour of dissolved FeSi constituents are governed mainly by inorganic soil and sediment materials. Each dissolved constituent behave by a characteristic way and depending highly on local environmental conditions. The environmental fate of FeSi and different grades of FeSi is for relevant parts connected to fate of its dissolution and transformation products. Dissolution, dissociation and speciation of dissociation products of FeSi is influenced by concentration of especially Si(OH) and Fe(II/III)-complexes, depending on grades, high Zr and Mg, Sr, Ba may occur.

Metals/elements can occur in different valences, associated with different anions or cations, and can be associated to adsorptive agents, such as Dissolved Organic Matter (DOM) in water, or bound to minerals in sediment or soil. Speciation highly depends on environmental conditions and chemistry and it makes the major differences in (bio) availability. Generally the adsorption of these constituents to organic materials is not very strong and adsorption to minerals is more pronounced.

SpecificFeSigrades (composition modifications) of ferrosilicon include periodic table IIA group alkaline earth metals. These grades include barium containing inoculant FeSiBa and strontium containing FeSiSr. These IIA constituents are predominantly in the II+ valency state cations in the environment. As all IIA group elements highly insoluble precipitates may be formed with some anions (e.g. sulphates) effectively limiting soluble background concentrations. This applies also for Fe (III) that forms insoluble hydroxide precipitates.

 

Silicon

Silicon is the second most abundant element after oxygen in the earth’s crust in the form of silicate minerals. Silicon is not known to be naturally present in the environment in its reduced elemental form. Normally Si in the environment is always bound primarily with oxygen as silica/silicic acid.

Silicon present in FeSi alloys exists both in Si (0) and Si (IV) oxidation states/forms. The released form is expected to be in the Si(IV) oxidation state. If released in the environment from FeSi in elemental form Si (0), it is rapidly oxidized and hydrolysed to Si (IV) silica species. The rate of these transformations is highly dependent on particle size/surface area of released silicon particles and environmental conditions.

Si(IV) in fresh water or seawater can occur in a number of chemical species, dissolved monomeric Si(OH)₄, dimerized, trimerized, colloidal or in the form of aggregated colloids of different physical size or entirely as insoluble particulate matter. Saturated monomeric concentration range upper limits are ca. 60-140 mg/l (temperature controlled). Dissolved silica may form precipitates with other elements like Al and Mg and may slowly form several types of clay minerals with these elements.

 

 

In dilute solutions and most typical environmental pH values silica is present as monomeric silicic acid Si(OH)₄. Since the dissociation constants of silicic acid is pKa₁ 9.9, pKa₂11.8, pKa_3,4 12 & 12 (measured at 30 °C) only a high pH (> 9) changes the molecule into ionic form.

 

If the concentration of silica is close to the standard solubility (at pH = 7.0-9.2) the fraction of dimers, with respect to the silicic acid, is not more than 1.0 %, fraction of trimers ca. 0.1 %, tetramers and low-molecular cyclic polymers (up to 6 units SiO₂) < 0.1 % (Weres et al. 1981).

 

Iron

Dissolved iron is present in the environment in two oxidation states Fe (II/III). Both states are non-volatile. Fe(II) “ferro” species are stable in anoxic and low oxygen conditions. Fe(II) is unstable in typical oxygenated water courses with a half-life of minutes – hours under favourable conditions, and oxidises easily to the Fe(III) state. The potential of the Fe(III)-Fe(II) couple, (0.77 V) is such that molecular oxygen can convert ferrous to ferric in acid solution or basic solution (Cotton et al., 1999).

Fe (III) “ferri” species are soluble and stable in water only at very low pH conditions. Normally Fe (III) reacts with water (hydrolysis) to form colloidal and insoluble ferric hydroxide Fe(OH)₃which in typical aquatic environmental conditions slowly precipitates to sediments.Fe (OH)₃is highly insoluble in water with K_sp= 1 x 10^-36(CRC Handbook). Formation of ferric hydroxide at pH levels above 5.0 limits the presence of iron in aqueous systems. Under conditions of very low oxygenconcentration, ferrous is freely soluble but ferric is not.

Heavy metals and organic matter may be strongly adsorbed to Fe precipitates. Fe (III) forms precipitates with phosphate. Iron ions, especially Fe(II) ions may be also adsorbed to dissolved organic material and some dissolved iron in natural waters may be present as soluble organic-complexes.

 

Cotton F.A., Murillo, C.A. and Bochmann, M.,1999. Advanced Inorganic Chemistry, 6th Edition; John Wiley and Sons, New York.

Weres 0., Yee A., Tsao L. (1981) Kinetics of Silica Polymerization. J. Coll. Interf. Sci., v. 84, No. 2, pp. 379-402.

Distribution modeling

The transport of metals between the aqueous phase and soil/sediment/suspended matter should be described on the basis of measured soil/water, sediment/water and suspended matter/water equilibrium distribution coefficients (Kd; also called partition coefficient, Kp).

Dissolved silica (and silicon) and all metals within FeSi alloys, are poorly volatile substances and partition predominantly in the aquatic or soil compartments. Calculated estimate of volatilisation behaviour of silicic acid from water to air gives Henry’s law constant value 6*10^-16atm*m³/mol indicating no volatilisation.

Precipitation (as minerals or pre minerals) with other ions/elements and the colloid forming behavior also play a significant role in the environmental fate of the constituents. These processes can not be simulated properly by general screening level fate models.

Coarse estimates of basic environmental behaviour of dissolved FeSi constituents could be given by applying multimedia fate models. Modeling is believed to be useless in trying to evaluate accurately distribution of the compounds which are major constituents of earth’s crust. Monitoring studies give normally sufficient and accurate information on partitioning behaviour of FeSi constituents in the environment.