Patent Application
20190276329
The disclosure relates to a method for producing calcium fluoride particles from gas streams containing gaseous hydrogen fluoride (HF).
Naturally occurring CaF2 minerals (e.g. fluorspar) are the principal source for anhydrous hydrogen fluoride (HF) used in the chemical industry. Hydrogen fluoride can be liberated from the mineral for example by the action of concentrated sulfuric acid:
CaF2+H2SO4→CaSO4+2HF
However, there is a need to find alternative ways of access to calcium fluoride to reduce the dependency from calcium fluoride mines.
Calcium fluoride is generated as a by-product of the purification of waste streams containing HF. Exhaust gases or waste streams containing HF are directed into an aqueous solutions containing calcium hydroxide upon which calcium fluoride precipitates. Such processes are described, for example in U.S. Pat. No. 3,743,704 and EP 0 536 051. However, the resulting CaF2 precipitates are too fine to be processed at industrial scale for the generation of anhydrous HF. To improve the handling properties of the precipitated calcium fluoride, organic precipitation agents (flocculants) can be added that increase the particle size of the precipitates. However, the particles then contain undesired organic contaminants. U.S. Pat. No. 4,120,940 and EP 2,952,478 A1 report the formation of CaF2 particles from an aqueous phase that are larger than 100 μm. However, the reaction still takes place in an aqueous phase. Such “wet” processes consume considerable amounts of water that has to be consumed and treated before it can be released back in the environment.
Some research was done in investigating “dry” conversion processes. Several studies report the generation of CaF2 in a dry process by directing an HF-containing gas over solid CaCO3 particles in a fixed bed reactor, for example: Yasui et al. describe in International Journal of Chemical Engineering, Volume 2012 (2012), No 6, page 1-9, http://dx.doi.org/10.1155/2012/329419. DOI 10.1155/2012/329419 ISSN 1687-806X, Yang et al, Wat. Res. Vol. 33, NO 16, pp 3395-3402, 1999, Yasue et al, CHISA 2008, Praha (2008), Yasui et al, AIChE Annual Meeting 2007, Salt Lake City, Paper No 173d (2007).
There is a need for generating CaF2 particles suitable for industrial conversion of CaF2 to anhydrous HF. Desirably the process employs HF gas generated from waste materials, preferably from waste materials comprising or consisting of partially fluorinated materials such as fluoropolymers.
Process for generating calcium fluoride particles having a particle size (d50) of from about 15 μm to about 300 μm, comprising contacting a gas stream comprising gaseous hydrogen fluoride (HF) with a fluidized bed comprising calcium carbonate particles in a fluidized bed reactor.
Before any embodiments of this disclosure are explained in detail, it is to be understood that the disclosure is not limited in its application to the details of construction and the arrangement of components set forth in the following description. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “containing”, “comprising,” or “having” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The use of “consisting of” is meant to be limiting to the items listed thereafter and equivalents thereof. The use of “a” or “an” is meant to encompass “one or more”. Any numerical range recited herein is intended to include all values from the lower value to the upper value of that range. For example, a concentration range of from 1% to 50% is intended to be an abbreviation and to expressly disclose the values between the 1% and 50%, such as, for example, 2%, 40%, 10%, 30%, 1.5%, 3.9% and so forth.
In the process of the present disclosure a gas stream containing gaseous hydrogen fluoride is fed into a reactor having a bed comprising solid calcium carbonate particles. The carbonate particles react with the HF and are converted into calcium fluoride particles. The process is carried out in a fluidized bed reactor. The bed comprising the calcium carbonate or consisting of calcium carbonate particles is fluidized by a gaseous fluidization medium, which means the particles are suspended in the gas flow of the fluidizing medium. The reaction conditions are chosen such that calcium fluoride particles result that are suitable for the industrial production of anhydrous hydrogen fluoride (AHF).
Typically, calcium fluoride particles suitable for the production of anhydrous hydrogen fluoride have a mean particle size (d50) of at least 15 μm, preferably at least 30 μm. The upper range preferably is up to about 300 μm, more preferably less than 200 μm, more preferably up to and including 100 μm. In a preferred embodiment the resulting particles have a mean particle size (d50) of from about 30 μm to 100 μm. The mean particle size (d50) is the median of a particle size distribution. It is the value where half of the population resides above and half of the population resides below this point. Calcium fluoride particles with a d50 of less than 15 μm are generally too difficult to handle and may react prematurely in the mixing equipment and blocking it. Particles with a particle size (d50) greater than 300 μm may react too slowly and the yield is not economical. Also, the resulting by-product CaSO4 contains too much unreacted CaF2 that it cannot be used as raw material in commercial applications.
In the process according to the present disclosure, the calcium carbonate particles convert into calcium fluoride particles by reaction with gaseous hydrogen fluoride (HF). The conversion is believed to start on the surface of the calcium carbonate particles and progresses from there to the core. Preferably, the conversion or purity of the resulting calcium fluoride particles, i.e. the CaF2 content, is greater than 90%, more preferably greater than 95%, and most preferably at least 97% (the percentages are weight percentages).
Suitable calcium carbonate particles for use in the process of the present disclosure include particles having a particle size (d50) of from about 35 to about 400 μm. Preferably, less than 10% (by weight), more preferably less than 5% by weight and most preferably less than 3% by weight of the calcium carbonate particles have a particle size of less than 3 μm. The CaF2 particles may shrink to some extent during the reaction with increasing conversion, which is believed to be the result of attrition between particles in the fluidized bed. Somewhat larger calcium carbonate particles may be used as starting material in case the reduction of the particle size is too great or the reaction can be stopped at a lower conversion rate. Also the flow rate of the fluidizing medium can be adjusted to control the attrition. The flow rate should be high enough to suspend the particles in the fluidizing medium and keeping them suspended thus forming a stable fluidized bed. Higher volume flows may destabilize the fluidized bed and may lead to greater attrition of the particles. Thus depending on the reactor design and reaction conditions the particle sized of the starting material may be adjusted. Generally, calcium carbonate particles having a d50 of at least about 25 μm can be used in the present process to provide CaF2 of from about 15 μm to about 300 μm particle size (d50). Preferred particle sizes of CaCO3 include ranges from about 25 μm to about 700 μm, more preferably from about 30 to about 300 μm and more preferably from about 40 to about 150 μm.
Preferably, the calcium carbonate particles have a purity of at least about 90% (weight percent). In one embodiment the calcium carbonate particles have a specific surface area (BET) of less than 10 m2/g, for example between 0.1 and 1 m2/g. The specific surface area (BET) can be determined using a Micromeritics ASAP 2012 analyser from Micromeritics GmbH, Aachen, Germany.
Suitable calcium carbonate particles are commercially available. The desired particle sizes can be obtained from sieved fractions obtained by grinding larger particles.
The calcium carbonate particles form the fluidized bed in the fluidized bed reactor where they are being converted into CaF2. The chemical composition of the fluidized bed thus changes from calcium carbonate towards calcium fluoride during the course of the reaction. Although a fluidized bed preferably consists of calcium carbonate particles at the beginning of the reaction it is also contemplate that the bed may comprise other particles, preferably inert particles that do not react under the reaction conditions and that can be separated from the generated calcium fluoride particles. Such particles may be used, for example, to influence the heat distribution in the reaction or to control the volume of particle flow.
The calcium carbonate particles are contacted by a gas stream containing gaseous HF. Preferably this gas stream is also the fluidizing medium suspending the calcium carbonate particles to form a fluidized bed. However, it is also possible that the HF containing gas stream is only part of the fluidizing medium and is present, for example as a mixture with one or more other gas streams. Preferred gas streams containing HF include those generated by the incineration of one or more fluorinated materials. Fluorinated materials include materials that are solid, liquid or gaseous at ambient conditions (20° C., 1013 hPa).
Suitable fluorinated materials include fluorinated hydrocarbons and substituted fluorinated hydrocarbons, i.e. fluorinated hydrocarbons that further contain atoms other than carbon, hydrogen and fluorine. Such other atoms may include chlorine, nitrogen, oxygen atoms and combinations thereof. In a particular embodiment, the fluorinated material comprises one or more fluorinated polymers, preferably partially fluorinated polymers. Suitable partially fluorinated polymers contain repeating units derived from fluorinated olefins having one or more carbon-carbon unsaturation, one or more fluorine and one or more hydrogen atom. Such polymers may have a molecular weight of more than 5,000 g/mole. Such polymers are typically solids. Suitable examples of partially fluorinated polymers include copolymers that have a backbone that is at least 30% by weight fluorinated, preferably at least 50% by weight fluorinated, more preferably at least 65% by weight fluorinated but also include hydrogen atoms. Examples of suitable partially fluorinated fluoropolymers include polymers and copolymers (i) comprising one or more partially fluorinated monomers, (ii) comprising one or more partially fluorinated monomers and one or more perfluorinated monomers, (iii) comprising one or more perfluorinated or partially fluorinated monomers in combination with one or more non-fluorinated monomers. Examples of perfluorinated monomers include fluorinated C2-C8 olefins that contain fluorine and chlorine atoms but no hydrogen atoms. Examples include tetrafluoroethene (TFE), hexafluoropropene (HFP); chlorotrifluoroethene (CTFE), 2-chloropentafluoropropene, dichlorodifluoroethene, 1,1-difluorethene, perfluorinated vinyl ethers (collectively referred to as PVE) and perfluorinated allyl ethers (collectively referred to as PAE). Examples of suitable perfluorinated allyl and vinyl ethers include those corresponding to the general formula
CF2=CF—(CF2)n—O—Rf (I)
In formula (I) n represents either 0 or 1. Rf represents a linear or branched, cyclic or acyclic perfluorinated alkyl residue containing at least one catenary oxygen atom. Rf may contain up to 8, preferably, or up to 6 carbon atoms, such as 1, 2, 3, 4, 5 and 6 carbon atoms. Typical examples of Rf include linear, branched alkyl residues interrupted by one oxygen atom, and linear or branched alkyl residues containing 2, 3, 4 or 5 catenary ether oxygens. Further examples of Rf include residues containing one or more of the following units and combinations thereof: —(CF2O)—, —(CF2CF2—O)—, (—O—CF2)—, —(O—CF2CF2)—, —CF(CF3)—, —CF(CF2CF3)—, —O—CF(CF3)—, —O—CF(CF2CF3)—, —CF(CF3)—O—, —CF(CF2CF3)—O—. Further examples of Rf include but are not limited to: —(CF2)r1—O—C3F7; —(CF2)r2—O—C2F5; —(CF2)r3—O—CF3; —(CF2—O)s1—C3F7; —(CF2—O)s2—C2F5; —(CF2—O)s3—CF3; —(CF2CF2—O)t1—C3F7, —(CF2CF2—O)t2—C2F5, —(CF2CF2—O)t3—CF3, wherein r1 and s1 represent 1, 2, 3, 4, or 5, r and s2 represent 1, 2, 3, 4, 5 or 6, r3 and s3 represent 1, 2, 3, 4, 5, 6 or 7; t1 represents 1 or 2; t2 and t3 represent 1, 2 or 3.
Suitable non-fluorinated comonomers include 1-chloroethene, 1,1-dichloroethene, and C2-C8 olefins such as ethene (E) and propene (P).
Examples of partially fluorinated monomers include fluorinated olefins that contain at least one hydrogen atom. Such olefins may additionally contain one or more oxygen atom and one or more chlorine atom. Specific examples of partially fluorinated copolymers include copolymers having a combination of monomers as follows: VDF-HFP, TFE-P, VDF-TFE-HFP, VDF-TFE-PVE, TFE-HFP, E-TFE-HFP, TFE-PVE, E-TFE-PVE and any of the aforementioned copolymers further including units derived from a chlorine containing monomer such as CTFE.
In a preferred embodiment the HF-containing gas is obtained by incinerating the fluorinated material. The incineration of the fluorinated material is preferably carried out in the presence of oxygen or a mixture of gases containing oxygen (incinerating gas). Depending on the starting materials and reaction conditions the oxygen content of air may be sufficient as incineration gas, otherwise additional oxygen may be added. The optimum conditions for incinerating the fluorinated materials may be determined experimentally by test reactions.
In the incineration reaction the partially fluorinated material typically gets mainly converted into CO2 and HF and H2O. The incineration reacts starts automatically in the presence of oxygen and the right temperature. However, it may be triggered by other incinerators or electrically, for example by generating sparks.
The ratio of fluorinated material and incinerating gas, i.e. the concentration of fluorinated material, will determine the HF content of the HF-containing gas produced. Other gases may be present if the fluorinated material contains other atoms than carbon, fluorine and hydrogen. For example, chlorine atoms and nitrogen atoms when present may result in the formation of HCl gas and gaseous nitrogen oxides. Nitrogen oxides may also be generated when air is used as incineration gas. No detrimental effect of nitrogen oxide on the conversion reaction has been observed. However nitrogen oxides should be retained in the exhaust treatment unit. The HF content of the gas may be lowered, if desired, by mixing the gas with one or more inert gases, for example but not limited to nitrogen, helium or other noble gases. The HF content of the HF-containing gas may include amounts of from 0.5 to 50% (volume percent). Preferably, the concentration (volume %) of HF in the gas stream is between 1 and 30%, more preferably between 3 to 15%. Preferably, the fluorinated material is composed such that approximately equimolar amounts of H and F are present in the fluorinated material. This may be achieved by mixing in other fluorinated materials if the fluorine content is too low, or by mixing in non-fluorinated hydrocarbons in case the amount of fluorine atoms is too high. Liquid and gaseous fluorinated or non-fluorinated materials may be used to adjust the H:F ratio in the fluorinated material to be incinerated.
Since H2O may be present in the gas stream, the gas stream is kept at a temperature and pressure that avoids the condensation of water to prevent the formation of aqueous hydrogen fluoride. Typically, the gas stream is kept at a temperature of greater than 100° C., preferably greater than 120° C. The incineration of fluorinated materials may be exothermic such that the temperature of the incineration reactor may have to be controlled.
In one embodiment of the present disclosure, the fluorinated material is a solid material. Preferably, its incineration is carried out in a fluidized bed reactor. The fluorinated material may be milled to have the appropriate particle size for forming a fluidized bed if necessary. A preferred fluorinated material comprises or consists of fluorinated polymers, more preferably of partially fluorinated fluoropolymers. The incineration gas may be used as fluidizing medium and the solid particles may form the fluidized bed. The incineration reactor is preferably made of steel with a high nickel content (e.g. 15 to 25 weight percent (wt. %) of Ni) and preferably a high content of chromium (e.g. from about 18 to 26 wt %). However, the incineration can also be carried out in a fixed bed reactor, such as for example but not limited to a rotary kiln or a gas incinerator, for example if a liquid or gaseous fluorinated material is used. In a preferred embodiment of the present disclosure the process is the treatment of fluorinated waste material to generate calcium fluoride. Therefore, in one embodiment of the present disclosure the process is a process of converting fluorinated waste material, in particular waste material comprising partially fluorinated polymers, into calcium fluoride.
The process of generating calcium fluoride particles will now be described by referring to
The HF-containing gas stream is directed out of the fixation reactor (3) via at least one outlet (8) and leaves the reactor as stream 1′. Stream (1′) contains a lower amount of HF than stream (1) because some or all of the HF has reacted with the calcium carbonate particles. The exiting gas stream (1′) may be directed to an exhaust treatment unit (9) as illustrated in
The exhaust treatment unit (9) may comprise one or more quenching units where the exhaust gas may be subjected to an aqueous basic solution to remove acidic residues. The treatment unit may comprise further units to remove solid particles from the gas stream and other impurities such as organic gases.
The fluorine fixation reactor (3) further contains at least one inlet (11) for entering the solid particles for forming the fluidized bed. This inlet may also be used for recovering the particles after the reaction, or a different inlet is installed for removing the CaF2 particles formed. Typically, the gas inlet is at the bottom section of the reactor and the gas outlet is at the head section of the reactor.
Typically, the fixation reactor and the HF-gas supply is kept at a temperature above 100° C. to avoid condensation of water and below the service temperature of the reactor material and material for the gas supply, and below the degradation temperature of the calcium carbonate particles. Typical suitable materials include polytetrafluoroethene (PTFE), which typically has a service temperature of up to about 200° C. Steels with a high nickel content may be used if higher temperatures are to be used, should this be desired. Suitable steels include steels having a nickel content of 15 to 25 weight %, preferably also having an additional chromium content of from about 20 to 30 weight %.
In one embodiment of the present disclosure also illustrated in
In one embodiment, the incineration reactor (2) is a fluidized bed reactor. However, fixed bed reactors or rotary kilns or other reactors known to be useful in incineration reactions of liquids or solids or gases may also be used. The incineration gas (24) can be used as fluidizing medium and the particles comprising the fluorinated materials (22) may form the fluidized bed. The gas flow is set such that the particles comprising the fluorinated material (22) are suspended and form the fluidized bed of the reactor. Typically, the gas inlet (20) is at the bottom section of the reactor and the gas outlet is at the head of the reactor (23). The gas stream (1) containing the gaseous HF is fed from the outlet (23) to the inlet (12) of the fluorine fixation reactor (2). It may passing through at least one heat exchanger unit (28) to bring the temperature of the HF containing gas stream (1) to the operating temperature in the fixation reactor (2) if required.
The set up used in examples 1 and 2 is illustrated in
The gas-stream 1′ exiting the reactor (201) was fed via an outlet at the head section of the first reactor (201) to an inlet at the bottom section of the second reactor (202). The same set up was applied to the other reactors (203-205) of the reactor cascade. The exhaust gas from a reactor could be fed either into another reactor or into the exhaust treatment unit (300). The gas supply to and out of the reactors contained several 3-way valves such that each reactor could be cut off from the gas supply and by-passed. In the exhaust unit (9) the gas was quenched (301) with basic water (solution of KOH), directed over a filter for solid particles (302) and then directed over a glass bead column (303) filled with a KOH solution in isopropanol.
For the experiments the reactor system was first purged with nitrogen. Then the reactors were heated to their operating temperatures (the incineration reactor was heated to 700° C. and the fluorine fixation reactors were heated to 150° C.). Then the fluorinated material (difluoromethane (DFM)) and air (incineration gas) were fed in the incineration reactor where the difluoromethane incinerated at an operating temperature of 700° C. The generated HF-containing gas stream (1) exiting the incineration reactor (101) through outlet (104) was cooled to a temperature between 150 and 200° C. in cooling unit (105) before it entered the fluorine fixation unit (200) due to the service temperature of the PTFE lining in the fixation reactors. In experiment 1 the HF content of the gas (1) entering the first reactor (201) of the fixation unit (200) was 5 vol % and 10 vol % in experiment 2. For experiment 1 the DFM volume flow was 10 Nml/min and in experiment 2 the volume flow was 20 Nml/min. The volume flow of air was 370 ml/min. The HF-gas flow (1) in the fluorine fixation cascade was adjusted that the particles were suspended to create a stable fluidized bed (at volume flows of about 4 and 16 mm/s). The retention times of the HF-gas in a fixation reactor were between 0.1 and 0.6 s. After the time intervals indicated in table 1 and 2 the gas entry to the first fixation reactor (201) was closed and the HF-containing gas stream (1) was fed directly to reactor (202). The bed was removed from the first reactor (201) and analysed.
Particle sizes of the CaF2 particles were determined by laser diffraction and expressed as (d50). A CILAS HR850 granulometer from CILAS ALCATEL was used. The CaF2 content (% conversion) was determined according to ISO 5439-1978.
The average particle size of the resulting CaF2 particles was 52 μm (d50) for the conversions below 50%. At conversions of greater 80% the particle size remained stable at 32 μm (d50).
The experiment for incinerating a fluorinated polymer was carried out in an incineration reactor similar to incineration reactor (2) illustrated in
| Number | Date | Country | Kind |
|---|---|---|---|
| 16170477.0 | May 2016 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2017/033469 | 5/19/2017 | WO | 00 |
