Patent Application
20110008235
The present invention relates generally to reutilization of ionic halides during production of elemental materials and more specifically, a method for moderate temperature reutilization of ionic halides.
Current processes for producing elements such as semiconductors, metals and metalloids create various by-products. Some of these by-products may be recycled within the process or may be purified and sold to other industries. However, some of the by-products may not have a large demand in other industries.
Industrial plant sizes for the production of elements, for example solar grade silicon (Si) or titanium (Ti) are expected to increase drastically over the next several years. One by-product in the production of elements by reduction of their halides, such as silicon tetrafluoride (SiF4), by reactive metals, such as sodium (Na), is ionic halides, such as for example, sodium fluoride (NaF). It is predicted that traditional markets for ionic halides such as the metallurgical industry, pharmaceutical industry, etc., may not be able to absorb such large production of ionic halides such as NaF. Furthermore, NaF is an alternative to hydrofluoric acid (HF), which is used to attack silicon dioxide (SiO2) and generate SiF4.
In one embodiment, the present invention relates generally to a method for reutilizing ionic halides in a production of elemental materials. The method includes reacting a mixture of an ionic halide, at least one of: an oxide, suboxide or an oxyhalide of an element to be produced and an aqueous acid solution at moderate temperature to form a complex precursor salt and a salt, forming a precursor halide from said complex precursor salt, reducing said precursor halide into said element to be produced and said ionic halide and returning said ionic halide into said mixture of said reacting step.
In one embodiment, the present invention is directed towards a method for reutilizing ionic halides in a production of a complex precursor salt. The method comprises forming an ionic halide during a reduction of a precursor halide to produce an element, recycling said ionic halide with a mixture of at least one of: an oxide, a suboxide or an oxyhalide of said element and an aqueous acid solution at moderate temperature and forming said complex precursor salt.
In one embodiment, the present invention is directed towards a method for reutilizing sodium fluoride (NaF) in a production of sodium fluorosilicate (NaSiF6). The method comprises forming said NaF during a reduction of a silicon tetrafluoride (SiF4) gas to produce pure silicon, recycling said NaF with a mixture of silicon dioxide (SiO2) and hydrochloric acid (HCl) solution at a moderate temperature and forming said NaSiF6.
So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.
A brief discussion of a process of producing high purity silicon from fluorosilicic acid will aid the reader on understanding a useful application of one embodiment of the present invention. An overall process 100 illustrated in
H2SiF6(aq)+2NaF(c)=Na2SiF6(c)+2HF(aq) Eq. (1)
The sodium fluorosilicate is filter dried in sub-step 114. Since the impurities with higher solubility than Na2SiF6 remain preferentially in the aqueous solution, the precipitation and filtration of Na2SiF6 results in a purification step beneficial towards the production of high purity silicon. Subsequently, the sodium fluorosilicate is thermally decomposed in step 116 with heat. In one embodiment, the sodium fluorosilicate may be heated up to temperatures in the range of 600 degrees Celsius (° C.) to 1000° C. The reaction equation for the thermal decomposition of sodium fluorosilicate is shown below by Eq. (2) and in sub-step 116 of
Na2SiF6(c)+heat=SiF4(g)+2NaF(c) Eq. (2)
The second major operation comprises the reduction of the precursor halide, such as for example silicon tetrafluoride (SiF4) gas, to an elemental material, such as for example silicon (Si), and an ionic halide, such as for example sodium fluoride (NaF). In one embodiment, the SiF4 is reduced by sodium metal (Na) as illustrated by a block of steps 120 in
SiF4(g)+4Na(s/l/g)=Si(s/l)+4NaF(s/l) Eq. (3)
The third major operation involves the separation of the produced elemental material, such as silicon (Si), from the mixture of the elements and the ionic halide, such as sodium fluoride (NaF), as shown in a block of steps 130 in
Previously, the ionic halide, for example sodium fluoride in the embodiment illustrated in
In one embodiment, an ionic halide or its aqueous solution, for example NaF, may be reacted with an aqueous solution of an acid, for example an acid of a halide, such as for example, hydrochloric acid (HCl) or hydrobromic acid (HBr), sulfuric acid (H2SO4), nitric acid (HNO3) or any organic acid such as acetic acid (CH3COOH), which sodium salts have a high solubility in, and at least one of an oxide, a suboxide or an oxyhalide of the element to be produced, for example silicon dioxide (SiO2) or an oxyhalide of Ti, V, Zr, Nb, Mo, Ta, W, U or Pu, in a vessel 202 via streams 220, 222 and 224, respectively. Hereinafter, oxide may be used to also refer to a suboxide or an oxyhalide where appropriate. The vessel 202 may be a reactor and may be heated. The materials of construction for the vessel 202 may be Teflon-lined steel, nickel or Inconel for temperatures of operation up to 150° C., and lead-lined steels for temperatures up to 250° C.
In one embodiment, the oxide of the element to be produced may be provided in small particles. For example, the particle size of the oxide may be from 100 nanometers (nm) to 1 centimeter (cm). In another embodiment, the particle size of the oxide may be from 1 micron (μm) to 1 millimeter (mm). In yet another embodiment, the particle size of the oxide may be from 1 μm to 50 μm.
The mixture of the ionic halide, the acid and the oxide react to form a complex precursor salt, for example sodium fluorosilicate (Na2SiF6) for Si production or sodium fluorotitanate (Na2TiF6) for Ti production, and a solution containing impurities. In addition, a salt or salt solution may be formed. The salt or salt solution may comprise at least one element from the ionic halide and at least one element from the acid. For example, in the example illustrated in
Referring back to the mixture, the reaction of the mixture may notably occur at a moderate temperature. In one embodiment, “moderate temperature” may be defined as being a temperature within a range of approximately 20° C. to 250° C. In another embodiment, “moderate temperature may be defined as being a temperature within a range of approximately 40° C. to 150° C. In yet another embodiment, “moderate temperature may be defined as being a temperature within a range of approximately 60° C. to 90° C.
In one embodiment, where the ionic halide is NaF, the acid is HCl and the oxide of the element to be produced is SiO2, the reaction to produce the complex precursor salt, for example Na2SiF6, is illustrated below by equations (4)-(7). Equations (4)-(6) illustrate the intermediate reactions and equation (7) illustrates the overall reaction.
NaF(aq)+HCl(aq)→HF(aq)+NaCl(aq) Eq. (4)
6HF(aq)+SiO2(s)→H2SiF6(aq)+2H2O(aq) Eq. (5)
2NaCl(aq)+H2SiF6(aq)→Na2SiF6(s)+2HCl(aq) Eq. (6)
4HCl(aq)+6NaF(aq)+SiO2(s)→Na2SiF6(s)+4NaCl(aq)+2H2O(aq) Eq. (7)
As illustrated by vessel 202 in
Referring back to the thermal decomposition at 206, the thermal decomposition may also produce a precursor halide, such as for example, silicon tetrafluoride (SiF4) via stream 230.
The SiF4 may be reduced at 212 to produce pure silicon out of stream 234 and NaF out of stream 232 similar to step 122 in
Alternatively, the solution containing SiF62− anions or the Na2SiF6 can be reacted with a strong acid, such as sulfuric acid (H2SO4), to generate SiF4 gas directly. This embodiment is illustrated below with reference to
Referring back to stream 232, the NaF produced from the reduction of SiF4 with Na may be recycled back into stream 220 to be reacted with a mixture of the SiO2 and HCl to produce more Na2SiF6. In one embodiment, the NaF provides a double source of fluorine ions to generate HF, which is used to attack the SiO2 and form SiF62− and sodium ions to obtain Na2SiF6 through precipitation of this solid with low solubility.
As noted above, the reaction between NaF, HCl and SiO2 may occur at a moderate temperature and may include some agitation or stirring. In one embodiment, moderate temperature may be defined, as noted above, as being a temperature within a range of approximately 20° C. to 250° C. Also, process 200 requires minimal raw materials to be introduced into the system, for example low cost SiO2 or sand that is readily available at minimal cost and some make-up NaF and HCl.
Although the
Similarly, the oxide of the element to be produced may be any oxide and not limited to silicon dioxide. For example, the oxide may include boron (B), aluminum (Al), silicon (Si), titanium (Ti), vanadium (V), zirconium (Zr), niobium (Nb), molybdenum (Mo), tantalum (Ta), tungsten (W), uranium (U), plutonium (Pu), or any Ti suboxide such as Ti3O5, Ti2O3 or TiO2-x, where x can be any real number between 0 and 1. For example, the oxide may be silicon dioxide (SiO2), titanium dioxide (TiO2) or a titanate such as calcium titanate (CaTiO3) or ilmenite (FeTiO3). Also included by example is an oxyhalide of Ti, V, Zr, Nb, Mo, Ta, W, U and Pu. The type of oxide may be determined by the desired type of elemental material that is to be produced. For example, if the process 200 is to be used to produce pure silicon, then SiO2 may be used. Alternatively, if the process 200 is to be used to produce pure titanium metal, then TiO2, Ti3O5, Ti2O3, TiO2-x, CaTiO3 or FeTiO3 may be used. Alternatively, if the process 200 is to be used to produce pure boron, then Na3BO3 may be used.
The precursor halide may also be any precursor halide and not limited to SiF4. For example, the precursor halide may include boron (B), aluminum (Al), silicon (Si), titanium (Ti), vanadium (V), zirconium (Zr), niobium (Nb), molybdenum (Mo), tantalum (Ta), tungsten (W), uranium (U) or plutonium (Pu). The type of precursor halide may be determined by the desired type of elemental material that is to be produced. For example, if the process 200 is to be used to produce pure silicon, then SiF4 gas or Na2SiF6 solid may be produced. Alternatively, if the process 200 is to be used to produce pure titanium metal, then TiF4 solid or Na2TiF6 solid, may be produced. Similarly, for the production of uranium, uranium tetrafluoride UF4 may be used. Similarly, the halide in the precursor halide may be any type of halide and is not limited to only fluorine (F). Other halides such as, for example, chlorides, bromides and iodides may be used.
The salt produced from the reaction in vessel 202 may be any salt depending on the ionic halide and the acid used and is not limited to NaCl. For example, when sulfuric acid (H2SO4) is used as illustrated in
The complex precursor salt may also be any type of halide complex salt and not limited to only Na2SiF6. For example, the precursor salt will also depend on the desired type of elemental material that is to be produced. For example, if the process 200 is to be used to produce pure silicon, then a fluorometallic compound such as Na2SiF6 may be used. Alternatively, if the process 200 is to be used to produce pure titanium metal, then a fluorotitanate such as, Na2TiF6, K2TiF6, CaTiF6 and the like, may be used.
The method 300 begins at step 302. At step 304, the method 300 reacts a mixture of an ionic halide, at least one of an oxide, a suboxide or an oxyhalide of an element to be produced and an aqueous acid solution at a moderate temperature to form a complex precursor salt and a salt. As defined above, moderate temperature may be a temperature within a range of approximately 20° C. to 250° C. The ionic halide, the oxide and the aqueous acid solution may be any one of the ionic halides, oxides and acids described above. The complex precursor salt and the salt may be any one of the complex precursor salts and salts described above.
The method 300 at step 306 forms a precursor halide from the complex precursor salt. For example, as described above, when the complex precursor salt is thermally decomposed, a precursor halide may be formed. The halide may be any one of the halides as described above.
The method 300 at step 308 reduces the precursor halide into the element to be produced and the ionic halide. As described above, the precursor halide may be reduced to form a desired element and the ionic halide. The element may be any one of the desired elements as described above.
The method 300 at step 310 returns the ionic halide into the mixture of the reacting step 304. Thus, the generated ionic halide formed from reduction of the precursor halide to produce a desired element may be reutilized or recycled within the process. The method 300 concludes at step 312.
The method 400 begins at step 402. At step 404, the method 400 forms an ionic halide during reduction of a precursor halide to produce an element. The ionic halide, the precursor halide and the element may be any one of the ionic halides, precursor halides or elements as described above.
At step 406, the method 400 recycles the ionic halide with a mixture of at least one of an oxide, a suboxide and an an oxyhalide of the element and an aqueous acid solution at a moderate temperature. The oxide and the aqueous acid solution may be any one of the oxides or acids discussed above. As defined above, moderate temperature may be a temperature within a range of approximately 20° C. to 250° C.
At step 408, the method 400 forms the complex precursor salt. As described above, the complex precursor salt may be produced from the reaction of the mixture of the ionic halide, the oxide and the aqueous acid solution at a moderate temperature. The method 400 concludes at step 410.
The method 500 begins at step 502. At step 504, the method 500 forms NaF during reduction of a silicon tetra fluoride (SiF4) gas to produce pure silicon.
At step 506, the method 500 recycles the NaF with a mixture of silicon dioxide (SiO2) and an aqueous hydrochloric acid (HCl) solution at a moderate temperature.
At step 508, the method 500 forms the Na2SiF6. The method 500 concludes at step 510.
The process 600 differs from the process 200 only in the step in which the precursor halide is generated from the complex precursor salt. The rest of the steps are as discussed above for the process 200.
In one embodiment, the complex precursor salt, for example Na2SiF6, may be filtered at 604, similar to 204 in
The ionic halide, the precursor halide, the oxide of the element to be produced and the element may be any one of the ionic halides, precursor halides, oxides or elements as described above.
In one embodiment, an ionic halide, for example NaF, may be reacted with an aqueous solution of a strong acid, for example sulfuric acid (H2SO4), and an oxide of the element to be produced, for example silicon dioxide (SiO2), in a vessel 702 via streams 720, 722 and 724, respectively. The vessel 702 may be a reactor and may be heated. The materials of construction for vessel 702 may be any of the mentioned above for vessel 202.
The mixture in 702 may produce a precursor halide, such as for example, silicon tetrafluoride (SiF4) via stream 728 and a salt, such as for example Na2SO4 via stream 726. Notably, in
In addition, the reaction may occur at a moderate temperature. In one embodiment, “moderate temperature” may be defined as being a temperature within a range of approximately 20° C. to 250° C. In another embodiment, “moderate temperature may be defined as being a temperature within a range of approximately 40° C. to 150° C. In yet another embodiment, “moderate temperature may be defined as being a temperature within a range of approximately 60° C. to 90° C. The precursor halide may be cleaned at 704 from other components, such as water, HF, SOF2 or Si2OF6, and reduced at 706 by a metal, such as Na, from stream 736 to produce the element, such as Si, out of stream 734 and an ionic halide, such as NaF, out of stream 732, which can be recycled back into stream 720.
The ionic halide, the precursor halide, the oxide of the element to be produced and the element may be any one of the ionic halides, precursor halides, oxides or elements as described above.
The synthesis of Na2SiF6 by reaction of NaF, HCl solution and SiO2 was performed. Stoichiometric amounts of the reactants (6:4:1 molar ratios of NaF, HCl and SiO2) were used. 7.0 g of SiO2 were added to a 200 mL aqueous solution of NaF and HCl that was previously heated at 80° C. Silica was used in the form of fine particles (crystalline quartz, Alfa Aesar, nominal surface area of 2 m2/g and average particle size of 2 microns). Right after adding silica to the solution, the temperature in the suspension increased by 7-10° C., due to the exothermic nature of the reaction, and the temperature dropped back to 80° C. in few minutes. The mixture was stirred at 80° C. for different amounts of time (0.25, 1, 2, 4 and 7 hours). After this time, the solids were recovered by filtration, washed with a limited amount of water and finally washed with methyl alcohol to expedite the drying process. Afterwards, they were dried in a convection oven, weighted and analyzed by means of X-ray diffraction (XRD) and thermogravimetric analysis (TGA). XRD did not detect any residual SiO2 in any of the recovered solids. We estimate that the yield was over 90%.
The synthesis of Na2SiF6 by reaction of NaF, HCl solution and SiO2 was performed. Stoichiometric amounts of the reactants (6:4:1 molar ratios of NaF, HCl and SiO2) were used. 7.0 g of SiO2 were added to a 200 mL aqueous solution of NaF and HCl that was previously heated at 80° C. Silica was used in the form of fine particles (crystalline quartz, 60 wt % of it over 100 microns and 40 wt % in the range 20-100 microns). In these embodiments, there was no measurable temperature increase. Since the silica surface available for reaction is much smaller, the reaction rate is also slower. The mixture was stirred at 80° C. for different amounts of time (0.25, 2 and 4 hours). After this time, the solids were recovered by filtration, washed with a limited amount of water and finally washed with methyl alcohol to expedite the drying process. Afterwards, they were dried in a convection oven, weighted and analyzed by means of XRD and TGA. The results showed that Na2SiF6 was formed with a yield, in the 4 hour experiment, of 54.3%.
The synthesis of Na2SiF6 by reaction of NaF, HCl solution and SiO2 was performed. Stoichiometric amounts of the reactants (6:4:1 molar ratios of NaF, HCl and SiO2) were mixed in 100 mL of aqueous solution. 3.6 g of SiO2 in the form of fine amorphous SiO2 particles were used (silica fumes, formed by agglomerates of particles with sizes below 400 nm). The mixture was stirred for 16 hours at approximately 24° C. After this time, the solids were recovered by filtration, washed with a limited amount of water and finally washed with methyl alcohol to expedite the drying process. Afterwards, they were dried in a convection oven, weighted and analyzed by means of XRD and TGA. The results showed that Na2SiF6 was formed with a yield of 85.4%
The synthesis of Na2SiF6 by reaction of NaF, HCl solution and SiO2 was performed. Stoichiometric amounts of the reactants (6:4:1 molar ratios of NaF, HCl and SiO2) were mixed in 100 mL of aqueous solution. 3.6 g of SiO2 in the form of coarse sand were used (typical particle sizes in the range 250-450 microns). The mixture was stirred for 8 hours at approximately 60° C. After this time, the solids were recovered by filtration, washed with a limited amount of water and finally washed with methyl alcohol to expedite the drying process. Afterwards, they were dried in a convection oven, weighted and analyzed by means of XRD and TGA. The results showed that Na2SiF6 was formed with a yield of 47.0%.
The synthesis of Na2SiF6 by reaction of NaF, HCl solution and SiO2 was performed. Stoichiometric amounts of the reactants (6:4:1 molar ratios of NaF, HCl and SiO2) were mixed in 100 mL of aqueous solution. 3.6 g of SiO2 in the form of coarse sand were used (typical particle sizes in the range 250-450 microns). The mixture was stirred for 16 hours at approximately 24° C. After this time, the solids were recovered by filtration, washed with a limited amount of water and finally washed with methyl alcohol to expedite the drying process. Afterwards, they were dried in a convection oven, weighted and analyzed by means of XRD and TGA. The results showed that Na2SiF6 was formed with a yield of 34.5%.
The synthesis of SiF4 was done by reaction of Na2SiF6 with H2SO4. 50 mL of concentrated H2SO4 (17.8 M) were loaded into a PFA flask. The flask was purged with He and then heated using a water bath to 80° C. under He flow. After this, 18.8 g of Na2SiF6 were quickly added to the flask maintaining the flow of He through the system. The exhaust gas mixture was passed through an ice-cooled trap to condensate moisture and afterwards was passed through a cold trap, which was cooled by means of liquid nitrogen in order to condensate SiF4. The evolution of SiF4 was monitored by controlling the weight gain of the trap at several times. The weight of collected product reached 80% of the theoretical weight in 10 minutes, 97% in 30 minutes and 100% in 45 minutes.
The synthesis of SiF4 was done by reaction of H2SiF6 with H2SO4. 50 mL of concentrated H2SO4 (17.8 M) were loaded into a PFA flask. The flask was purged with He and then heated using a water bath to 80° C. under He flow. After this, 50 mL of H2SiF6 (20-25 wt %) were quickly added to the flask maintaining the flow of He through the system. The exhaust gas mixture was passed through an ice-cooled trap to condensate moisture and afterwards was passed through a cold trap, which was cooled by means of liquid nitrogen in order to condensate SiF4. The evolution of SiF4 was monitored by controlling the weight gain of the trap at several times. The weight of collected product reached 92% of the theoretical weight in 10 minutes, 95% in 35 minutes and 96% in 50 minutes.
While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of a preferred embodiment should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.
