FR2611195A1 - Process for the purification of and overpressurisation of electrolytic fluorine - Google Patents

Abstract

Process for the purification of and overpressurisation of electrolytic fluorine containing impurities of MF.nHF (M being an alkali metal or NH4) which makes it possible to directly recycle the effluents obtained to the electrolysis, characterised in that the fluorine is purified with the aid of and overpressurised in the presence of anhydrous hydrofluoric acid optionally containing one or a number of fluorides of MF type in solution, at low temperature.

Description

IFn & Ioe AND NISI PROCEDURE W ELECTROLYTIC FLOOD PRESSURE
TECHNICAL AREA
The invention relates to an industrial process for purifying and simultaneously putting slight overpressure of fluorine at the outlet of production electrolytic cells.

STATE OF THE ART
Fluorine is obtained by electrolysis of hydrofluoric acid introduced into a molten bath of KF-2HF. Hydrogen and fluorine are recovered separately at the top of the electrodes immersed in the electrolyte.

Fluorine entrains particles of electrolytic bath in the form of a compound of variable composition KF, nHF, n which can vary from 0.5 to 10.

The physical state of this impurity, 'which is akin to a solution of
KF in HF, varies from solid1 to pasty or liquid, and depends on both the value of n and the temperature. Fluorine also contains noncondensable gases such as nitrogen from the purging of piping installations.

The presence of KF, nHF impurity is very annoying because, in addition to the chemical disadvantages that this can represent for certain uses of fluorine, condensations can occur in pipes, valves, regulating organs ..., and cause very serious disturbances (blockages, corrosions ...), even dangerous for the operation of the installations: risks - of explosion, untimely shutdown of the electrolysis causing delicate restarts ...

Usually, the purification is carried out by passing the fluorine leaving the electrolysis, directly into a surface condenser, tubular, supplied with refrigerant at around -800C, then in a tower packed with NaF crystals playing a double chemical role and physical, absorption and filtration of impurities.

Since the surface exchanger does not directly provide sufficient purification, this necessitates then passing the fluorine through a bed of crystals of said NaF. The latter introduces pressure drops which can be significant and difficult to control. It must be regenerated periodically by heating to evacuate the hydrofluoric acid retained.

From time to time, it must be changed and then constitutes a fluorinated effluent which must be treated and then rejected. Periodic regeneration leads to the need to have two installations in parallel operating alternately. Knowing that the fluorine electrolysis technology is such that only small pressure drops and very small pressure variations in the fluorine circuit can be tolerated, this process is not very satisfactory from this point of view and requires the use of bulky devices (with large surfaces) and important precautions so that when changing installations, valve operations do not cause pressure variation in the fluorine circuit. These constraints do not facilitate the operation or the maintenance of the installations.

Another method consists in passing the fluorine through a crystallizer cooled to -1400C so as to retain in solid form hydrofluoric acid and the fluoride particles in solution. The use of two parallel devices operating alternately is still necessary, bringing about the drawbacks which have just been described.

In addition, in these two methods, surface heat exchangers are used whose efficiency is insufficient.

As regards the overpressure of fluorine, it is only possible with devices whose sealing members resist both fluorine, low temperatures and mechanical stresses, for example membrane pumps. But their use is delicate.

OMIT OF THE INVERTION
The object of the invention is a high-performance fluorine purification process, that is to say at least equivalent to what is usually obtained, which can operate continuously without periodic shutdown, introducing low pressure drops in the fluorine circuit; this results in simplified operation, thanks in particular to the fact that the device used is self-cleaning.

Another object is a process for purifying fluorine which can be combined with simultaneous overpressure of said fluorine.

Another object is a process for purifying fluorine which makes it possible to use apparatuses for pressurizing fluorine whose sealing parts subjected to mechanical stresses are not in contact with it, or to use apparatuses for setting in pressure using liquids, such as liquid ring pumps or ejectors.

Another object is a process for purifying fluorine having an improved yield during the exchange of frigories.

Another object is a process for purifying fluorine that is both more economical and simple to implement and requires less bulky installations.

Another object is a process giving directly recyclable effluents.

DESCRIPTION OF THE IHVERTION
The invention is a simplified industrial process for purifying and overpressuring fluorine leaving the electrolytic cells and containing impurities constituted by MF, nHF (M representing an alkali metal or the ammonium ion), in the form of particles. in suspension, said process making it possible to directly recycle the effluents obtained by electrolysis; it is characterized in that the fluorine is treated by and overpressed in the presence of a treatment liquid consisting of anhydrous hydrofluoric acid optionally containing one or more fluorides of MF type in solution, at low temperature.

The fluorine leaving the electrolysis cells contains, as impurities, vesicles or particles in the liquid, pasty or solid state depending on the value of n, that is to say their respective contents of MF and HF.

The solid form corresponds to a high proportion of MF, which can go up to pure MF, for the same temperature; the pasty or liquid forms correspond to lower contents of MF, which can go up to pure HF, or at higher temperatures.

These particles are removed by a treatment carried out at very low temperature with the aid of anhydrous liquid hydrofluoric acid optionally charged with one or more fluorides of the MF type, treatment which operates a washing of the gas and its cooling resulting in trapping and intense condensation. of all the particles present.

The treatment liquid previously refrigerated at very low temperature is introduced into the gas to be purified by any known means (spray nozzle, spray bars, venturi, etc.) generally counter-current to the gas. The treatment is also carried out in all types of known gas washing devices (tray towers, packing ...).

The entire device also functions as a very efficient mixing exchanger for transferring frigories from liquid to gas.

According to the invention, fluorine can also be treated to be purified and simultaneously conveyed yeah under overpressure.

The fluorine is transported either usually under the action of the pressure generated in the electrolysis tank, most often helped by any pressurization device arranged downstream on the purified gas circuit as is known from the prior art , or preferably according to the invention using an overpressure device operating by means of or in the presence of the treatment liquid, for example ejector, liquid ring pumps or the like, rotoclones, turbines, etc. In this case , the pressurizing apparatus is also and simultaneously the device for purifying and cooling the gas according to the invention; in general, all gas propellants, operating in the presence of liquid, are suitable provided that their sealing members subjected to mechanical stresses are protected from the gas by the treatment liquid, and that they can work in the presence of said liquid.

It is thus seen that the pressurization of the fluorine can be subsequent to or preferably simultaneous with its treatment with the treatment liquid.

The treatment liquid obtained after gas treatment contains a higher amount of NF in solution from the particles killed; it can be recycled as a treatment liquid and / or as an electrolyte after adjusting its fluoride composition.

The treatment liquid can be anhydrous hydrofluoric acid used alone or containing in solution at least one of the fluorides of the group called M, consisting of alkali fluorides and ammonium fluoride. Preferably, solutions of KF in HF, or solutions of NH4F in HF or of NR4F and IF in HF are used, or better still the electrolysis bath itself.

The treatment liquid can be used in a single pass in the installation, in this case it is loaded with dissolved fluorides thanks to the slaughtered particles and is recycled directly to the electrolysis.

However, it can also be recycled in said installation as many times as necessary to bring it to a determined concentration of dissolved fluorides.

In general, it is preferable to limit oneself to a concentration such that the speed of dissolution of the fluorides in the washing liquid is instantaneous and that there is no risk of crystallization due to the temperature. For example, it is considered that up to a content of approximately 10 Z (weight) of IF in HF, this dissolution is instantaneous.

In case of recycling of the liquid in the installation, it is obviously necessary to provide a purge of washing liquid and a supply of regular fresh liquid; they can be continuous or sequential;
The treatment temperature is as low as possible, it is limited by the crystallization of the treatment liquid. Thus, when hydrofluoric acid is used alone, undergoing a single pass or sufficiently little recycling in the treatment installation so that its fluoride concentration is low, one operates at a temperature as close as possible, by higher values, of -840C crystallization point of said acid; when using a solution of HF and NF fluoride, the temperature is limited by the crystallization point of this solution, it is obviously advantageous to be in the vicinity of the eutectic compositions which correspond to the lowest temperatures. In practice, when using HF alone or dilute fluoride solutions, it is operated at temperatures between -40 C and -840C and preferably between -800C and -840C. We can lower the operating temperature and then operate , at temperatures between -600C and -100 C and preferably between -840C and -1000C when using a treatment liquid sufficiently charged with fluorides of type NF and in particular KF.

The treatment liquid can be cooled to its operating temperature by any known device, for example using an exchanger itself supplied with refrigerant. But of advantageous fan, one can recover the frigories available in the purified fluorine to cool using a temperature exchanger the washing liquid before its introduction into the treatment device or directly fluorine during its treatment.

After being purified and possibly simultaneously pressurized, the fluorine passes through a condenser and / or a liquid-gas separator, then optionally through a demister of all known types, such as inert filters, fiber mat ...

The installations necessary for implementing the process can be made of any material resistant to the products used (F2, HF, MF) and to very low temperatures. Thus, metals such as resilient steels (stainless steel), synthetic materials (teflon), composites, ceramics can be used.

This process can be carried out on a fluorine collector as soon as it leaves the electrolysis tanks; thus given the simplicity of the process and the device, a minimum of control members and valves are located upstream of said device on the unpurified fluorine and present risks of clogging; all of the pressure, flow, transfer, safety and temperature exchange regulation devices are located downstream of the installation on the purified fluorine, and thus make it possible to avoid any risk of clogging.

The method according to the invention can be implemented according to several modes: purification only with the aid of the treatment liquid operating the removal of the particles and the cooling of the gas, this purification being possibly followed by a pressurization of the fluorine refined; purification and simultaneous overpressure of fluorine according to the invention; a first purification step followed by a second purification step and simultaneous pressurization of the fluorine.

A first mode of application of the process for the purification and simultaneous overpressure of fluorine is illustrated in FIG. 1.

The device for purifying and simultaneously boosting the fluorine consists of an ejector P12, the outlet of which is connected directly to a tank R2 containing a flywheel of recycled washing liquid (for example anhydrous HF or solution of IF in HF such as the electrolysis bath) and serving as a separator-contender.

The Pîl pump takes the washing liquid in R1, makes it pass through an exchanger El itself supplied with refrigerant, before bringing it to the ejector P12 where it serves as carrier fluid and washer sucking in the fluorine arriving from the electrolysis in 14. The purified fluorine arrives surprises in the tank-washer-separator R1, is evacuated in 16 after having passed through a demister of D1. The supply of fresh anhydrous hydrofluoric acid is made in 15 using a manual or automatic valve, regulated
RE or remote-controlled; it can be continuous or periodic.

Likewise, the purging of washing liquid consisting of NF fluorides (for example KF) dissolved in HF is carried out at 27 downstream of the Poil pump. This withdrawal can be controlled using a manual or automatic valve, regulated RE, controlled or remotely controlled; it can be continuous or periodic. In particular, it can be slaved to a timer or to the liquid level in R1, or to any other parameter.

We see that in this case, the treatment liquid is recycled; its operating temperature obtained in the exchanger El is as low as possible, but must not cause crystallization of the transported liquid. The treatment liquid may also not be recycled, in this case the fresh acid arrives directly at El and the used acid is completely evacuated at 27.

In the case where the treatment liquid is anhydrous HF alone or having a low concentration of KF, the temperature is adjusted between -40 and -840C, and preferably between -80 and -840C. If the concentration of
IF is higher, the temperature can be adjusted to a value between -400C and -1000C and preferably between -600C and -100 C. We see in this case that the frigories evacuated with fluorine are not recovered.

A second mode of application of the two-step process is illustrated in FIG. 2.

In this case, the fluorine charged with the particles to be purified arrives from the electrolysis at 4, is introduced in a first step into a column.
C of washing of any known type where it is cooled using the exchanger El, in which the purified and cold fluoride circulates, then treated in L by the treatment liquid using any known device, brought by pump P1 and regulated by valve 3; the exchanger El is continuously cleaned by trickling. At this stage, the fluorine has undergone a first stage of intense cooling, and most of the particles MF, nHF are broken down and pass into solution in the treatment liquid. The latter which is loaded with fluorides is recovered at the bottom of the column and evacuated to tank A.

In a second step, the treated fluorine arrives in the ejector P2 supplied in addition with washing liquid recycled from R via a pump P1 and a temperature exchanger E, itself supplied with refrigerant, intended to bring it to the desired temperature. A valve 2 makes it possible to regulate the flow.

As in the case of FIG. 1, the fluorine-treatment liquid mixture arrives in the condenser-separator R under overpressure; the washing liquid is condensed there at low temperature and separated from the gas before recycling.

The supply of fresh anhydrous hydrofluoric acid is for example at 5 directly into the tank R, and the purging of the loaded treatment liquid is at 7. Although not shown, recovery of the frigories conveyed by this flow can be made , for example by cooling the flow 5.

As for the fluorine, it is evacuated through a demister to be used directly or after having yielded its frigories to the fluorine returning to El, as described above.

In this mode of application, we see that we operated in two stages first a washing and a condensation at low temperature, then a second cooling and washing at a lower temperature to improve the condensation of the washing liquid in R , while avoiding the risks of crystallization of said liquid, with simultaneous movement of the fluorine by suction-overpressure.

In the two illustrated application modes, the fluorine pumping devices P12 and P2 can be any type of pump or gas booster device capable of operating in the presence of the refrigerated treatment liquid.

By way of example, the process according to the invention can be carried out with the following operating parameters, with reference to the mode of application of FIG. 2 applied to KF, nHF impurities purified with a washing liquid consisting of Anhydrous HF containing at most 10 Z (weight) of KF.

P2 supply of washing liquid
1st setting: 20 m3 / h at 1.1 bar (relative)
20 setting: 40 m3 / h at 1.3 bar (relative)
In 4: fluorine flow rate: 100 Nm3 at atmospheric pressure approximately (relative O bar)
temperature: 250C
HF content: 4 Z weight
IF: 5 ppm weight
Fluor out of C
temperature: -400C
HF content: 4 Z weight
IF: traces
Fluorine recovered in R (after condensation of HF)
temperature: -800C
HF content: 1.5 7. weight
IF: same content as on leaving C
HF vesicle: 5 ppm by weight.

Fluorine recovered in 6 (after D and El)
flow rate s 100 Nm3 at: 0.1 bar (relative) for setting 1
0.3 bar (relative) for setting 2 HF and KF content s same content as the fluorine recovered in R vesicle content s trace.

Claims (9)

1. Process for the purification and pressurization of electrolytic fluorine containing impurities of the MF, nHF, M type representing at least one of the elements of the group consisting of alkali metals and ammonium ion, allowing direct recycling to electrolysis of the effluents obtained, characterized in that the fluorine is treated with and surprises in the presence of a treatment liquid consisting of anhydrous hydrofluoric acid optionally containing one or more fluorides of the MF type in solution, at low temperature. 2. Method according to claim 1, characterized in that the purification and the pressurization of the fluorine are simultaneous.  3. Method according to any one of claims 1 and 2 "characterized in that the fluorine is treated by all types of gas propellants operating using or in the presence of the treatment liquid and including the sealing members subjected to mechanical stresses are protected from fluorine by said liquid, and preferably by ejectors or pumps with liquid rings. 4. Method according to any one of claims 1 to 3, characterized in that the temperature of the treatment liquid is between -400C and -840C and preferably between -800C and -840C when said liquid is slightly loaded with fluorides of type NF. 5. Method according to any one of claims 1 to 3, characterized in that the temperature of the treatment liquid is between -600C and -1000C when said liquid has a high content of fluorides of type NF and preferably IF. 6. Method according to any one of claims 1-3 to 5, characterized in that the method comprises a first purification step and a second complementary purification step at a lower temperature and simultaneous overpressure. 7. Method according to any one of claims 1 to 6 characterized in that the treatment liquid is an RF-HF solution. 8. Method according to any one of claims 1 to 7 characterized in that the purification and the pressurization are followed by a liquid-gas separation operation. 9. Method according to any one of claims 1 to 8, characterized in that the frigories available in the purified fluorine serve to lower the temperature of the fluorine entering the process.