WO2008152117A2

WO2008152117A2 - Process and apparatus for reducing arsenic in an acidic liquid1

Info

Publication number: WO2008152117A2
Application number: PCT/EP2008/057437
Authority: WO
Inventors: Gerard Gasser; Yves Santa Eugenia; Markus Hasenbohler
Original assignee: Buss ChemTech AG
Current assignee: Buss ChemTech AG
Priority date: 2007-06-12
Filing date: 2008-06-12
Publication date: 2008-12-18
Anticipated expiration: 2009-12-12
Also published as: WO2008152117A3; AR066976A1; TW200914377A; GB0711302D0

Abstract

A process for reducing arsenic in an acidic liquid comprising arsenic in an oxidized form, wherein the liquid is subjected to electrolysis in a single step and in a single compartment electrolytic cell using pulsed current in the presence of a conduction salt. A reference electrode comprising an electrolyte bridge made of PTFE is also discussed.

Description

PROCESS & APPARATUS

The present invention relates to a process and apparatus for the removal, recovery, or the removal and recovery, of arsenic. More particularly, it relates to a process and apparatus for the removal, recovery, or the removal and recovery, of arsenic from industrial waste waters, natural waters and other liquids. Most particularly, it relates to a process for the removal, recovery, or the removal and recovery of arsenic from hydrofluoric acid.

Anhydrous hydrogen fluoride is generally produced by reacting a mixture of fluorspar and sulphuric acid. Impurities produced in the reaction include silicon tetrafluoride, sulphur dioxide, non- volatile acids such as H₂SO₄ and H₃PO₄, water and arsenic. The desired product, hydrogen fluoride, is conventionally purified by distillation. However, the distillation process does not enable the arsenic to be separated such that the product anhydrous hydrogen fluoride still contains unacceptable quantities of arsenic.

The anhydrous hydrogen fluoride made by conventional methods generally contains from about 20 to about 600 ppm of arsenic depending on the arsenic contained in the fluorspar ore. As anhydrous hydrogen fluoride is used as a fluorinating agent in the production of organic and inorganic chemicals, the presence of the arsenic, which is a catalyst poison, can effect the efficiency of the processes in which the hydrogen fluoride is used, and will result in a reduced catalyst life, hi addition, the presence of arsenic in the feed and/or product streams can cause severe corrosion of the process equipment.

Dilute hydrogen fluoride has a variety of uses. For example, it is used in metal pickling and glass etching. Although the presence of the arsenic does not have a deleterious effect on these processes, the arsenic will be present in the effluent and thus expensive treatments have to be carried out on waste streams to remove the arsenic. Unfortunately, known treatments are costly.

In addition to being found in fluorspar, arsenic is also present in phosphate rock. When phosphate rock is treated with sulphuric acid to produce phosphoric acid, the arsenic remains as an impurity in the product phosphoric acid. The arsenic then has to be removed particularly if the phosphoric acid is intended for use in food. Fluorosilicic acid (H₂SiFo) is produced in significant quantities as a waste product during the conversion of apatite, which is a phosphate rock, to phosphoric acid. However, volatile arsenic products such as AsF₃ are present in the fluorosilicic acid. As fluorosilicic acid is used for fluorinating drinking water, it will be understood that it is essential that the arsenic is removed. Fluorosilicic acid is also used as a starting material in the production of hydrofluoric acid.

The water regulations introduced in the USA in 2006 require that the amount of arsenic present in water is 10 ppb or less. Prior to the introduction of these regulations, the maximum amount of arsenic allowed to be present in water was 50 ppb. Thus the requirement to remove the arsenic from natural or industrial waters is extremely acute and becoming more so.

It will therefore be understood that it is desirable to provide a process which will enable recover)'' and/or removal of arsenic from any liquid that may contain it. Such liquids include, but are not limited to: hydrofluoric acid; fluorosilicic acids such as H₂SiF₆ and HSiF₅; phosphoric acids such as H₃PO4; water from any source such as ground water, river water, and rain water; lixiviates from dumps and mines; and all public and industrial effluents.

Various processes have been proposed for removing arsenic from water, particularly that intended for use as drinking water, or from industrial effluents.

The known processes can be split into physical processes, chemical processes, biological processes, and electrochemical processes.

Physical processes include membrane processes such as nanofiltration, reverse osmosis, electrodialysis and sorption processes. Sorption processes include those which use activated alumina or ion exchange resins. Whilst these processes go some way to removing arsenic, they all have the disadvantage of producing a toxic liquid effluent which must be treated before disposal. In this connection, it should be noted that prior art to disposal, arsenic wastes should be neutralised and stabilised. Even after such treatment, the disposal must be in dedicated dumps for toxic wastes. The cost of these processes and the subsequent disposal is high. Chemical processes include those in which, for example, oxygen, ozone, chlorine, fluorine, hypochlorite, permanganate, Fenton Reagent or hydrogen peroxide are used as oxidation agents. As with the physical processes, these have the disadvantage of leaving toxic wastes which must be treated for safe disposal. Further, if these chemical processes are used in combination with the production of anhydrous hydrogen fluoride, a further distillation of the product is required downstream. Alternative processes include co-precipitation with iron or aluminium. These processes also produce toxic waste.

Biological processes comprise the biological treatment of, for example, water using a cocktail of microbes. These microbes are generally specifically grown and when used precipitate toxic compounds such as arsenic. The main disadvantage of these biological processes is that they produce a toxic sludge which has to be treated for safe disposal. A further disadvantage of the biological processes is that they cannot be used in acidic environments.

Known electrochemical processes for the removal of arsenic comprise oxidation of trivalent arsenic, As(III), into pentavalent arsenic As(V). Where the composition to be treated is a highly concentrated acid that has very low electrical conductivity, a molten salt conducting bath, is used.

US 5108559 and US 5100639 describe electrochemical processes for the removal of oxidizable impurities, such as arsenic, from anhydrous hydrogen fluoride. This process uses an electrolysis cell operated under direct or alternating current at a temperature and pressure which ensures that the anhydrous hydrogen fluoride is in the liquid state. During the electrolysis, the impurities present in the hydrogen fluoride are oxidized. In particular, arsenic is oxidized into a nonvolatile form which remains in solution. One disadvantage of this process is that the anhydrous hydrogen fluoride has to be distilled and a toxic liquid waste, which must be treated or carefully disposed of, is produced.

As anhydrous hydrogen fluoride has a veiy low conductivity, where a direct current is to be used for the process, US 5108559 and US 5100639 teach that an electrolyte such as potassium fluoride, or other alkali metal fluoride or water should be present. In any event, electrolysis under direct current conditions is inefficient and the yields are low unless electrolyte salts or water are added.

Although the process is more efficient if an alternating current is used, fluorine losses occur and the residue of the distillation contains hydrogen fluoride and fluorinated arsenic compounds such as HAsF₆. This residue has to be stabilized and disposed of in a special dump for toxic waste.

An alternative process is described in EP 0480254. Here a process is described in which anhydrous hydrogen fluoride is subjected to electrolysis by the application of what is assumed to be direct current. The reaction is carried out in an inert gas atmosphere to prevent the accumulation of potentially explosive gases. This process has the disadvantage of requiring complicated and costly scrubbing of the off-gas which contains not only fluoride but also arsenic. The reaction requires the application of a high voltage, up to 50V. Thus the power consumption for the reaction is high making the process relatively expensive. Further, as a high voltage is used and as the process has to be run for a relatively long period of time, there is a risk that the anhydrous hydrogen fluoride will decompose into H₂ and F₂ which will eventually result in fluoride loses. As with other processes, there is also the problem of the co-production of toxic liquid and gaseous effluents which must be treated prior to disposal.

US 5599437 describes an electrochemical process in which a pulsed current of from 0.5 to 1000 Hz, preferably between 10 and 100 Hz, and a duty cycle of 50% or less, preferably between 10 and 50%, is used. The process is applicable to solutions which contain electroactive chemical species such as metallic ions. The cathode is configured such that it has a large surface area. The cathode is made of a packed bed of carbon particles of 1 mm diameter or of a specific surface of 80 to 1500 m /g. The cathode has a roughness factor of at least about 10,000 and is optionally coated with an ion exchange resin such as perfluorinated sulphonic acid ionomer resin. Metals deposited are copper, silver, gold, zinc, nickel, mercury, lead, uranium, cadmium and chromium. However, the process is not applicable to arsenic due to its high oxidation level. In addition, the process cannot be used in highly concentrated acidic mediums.

Thus all of the known processes require the use of additional reagents such that waste streams are produced which have to be treated or be subjected to careful disposal. Even the electrochemical processes, which are based on oxidation, leave toxic wastes which have to be treated.

It is therefore desirable to provide a process which enables arsenic to be removed from liquids in such a way that there is no residual toxic waste to be treated, hi particular, it is desirable to provide a process in which the arsenic is recovered in metallic form since it is a commercial product which can then be used in various industries.

Thus according to the present invention there is provided a process for reducing arsenic in an acidic liquid comprising arsenic in an oxidized form, wherein the liquid is subjected to electrolysis in a single step and in a single compartment electrolytic cell using pulsed current in the presence of a conduction salt.

By "acidic" we mean that the liquid has a pH of from 0 to 8.

Thus, any As(III) in the liquid being treated will be reduced in accordance with the following reaction:

As(III) + 3 e^" → As

Any As(V) present in the liquid being treated will be reduced to As(III) when is then further reduced to As metal as detailed above, however this is all as a single step reaction. The conduction salt also acts to depolarize the anode and this minimises, or preferably prevents, the re-oxidation of any As(III) to As(V) at the anode.

Thus, in the process of the present invention the equilibrium of the redox system is displaced in favour of the thermodynamically least stable species As metal. Similarly the process favours the slowest reaction, i.e. the reduction, rather than the kinetically fast reaction.

It will be understood that one benefit of the present invention is that as the arsenic is reduced to the metal state it can be readily isolated and recovered. The arsenic in this state is a commercial product which can be recycled for industrial use, Such uses include doping in the microelectronics industry. The ability to recover and subsequently use the arsenic means that the process of the present invention is particularly economically attractive.

A further benefit of the present invention is that there is no waste produced, other than the desirable arsenic metal, which has to be treated or disposed of safely.

The process of the present invention may be applicable to the removal of arsenic at any concentration in the liquid being treated such as from several tens of percent such as 20 percent to trace amounts such as parts per trillion. The liquid may itself be conductive. However, due to the presence of the conduction salts, liquids which have low, or very low, levels of conductivity can be subjected to the process of the present invention.

hi a preferred aspect of the present invention, the process is carried out in the presence of a reference electrode.

Any suitable conduction salt may be used. Conduction salts having a redox potential of from about 0.6V to about -1.8V compared with the Normal Hydrogen Electrode (NHE) are particularly preferred. Suitable salts include hydrogen peroxide and hydrazine derivatives such as hydrazine fluoride, N₂H₄F, hydrazine sulphate, hydrazine nitrate, hydrazine phosphate and hydrazine hydroxide with hydrazine fluoride being particularly preferred.

In general, the hydrazine salt will be selected such that it is appropriate for the treated medium and will leave no additional impurity. Thus hydrazine fluoride is ideal where the substance being treated is hydrogen fluoride, hydrazine phosphate is ideal for H₃PO₄, hydrogen sulphate is ideal for H₂SO₄ and so on. Without wishing to be bound by any theory, it is believed that the conduction salt increases the conductivity of the medium in which the electrolysis is to take place. Increasing the conductivity of the medium is important to maintain the redox potential in the As(III) stable zone according to the Pourbaix diagram.

In addition, the conduction salt acts to depolarise the anode thereby preventing unwanted oxidation reactions occurring there.

A further benefit attributable to the conduction salt is that it does not contaminate the solution with impurities, nor does it produce liquid or gaseous toxic waste. This is because the conduction salt is consumed during the electrolysis and therefore disappears from the liquid at the end of the operation. The reaction of the present invention preferably provides products which are neither toxic nor undesired. Thus, the process does not cause toxic substances to be released into the environment. For example, when N₂H₄F is the conduction salt the decomposition products will be nitrogen which is volatile and hydrogen fluoride which remains in the reaction mixture where the liquid being treated is hydrogen fluoride.

Any suitable amount of conduction salt may be added to the liquid being treated. In a preferred arrangement, the quantity of conduction salt added is dosed in such an amount that It is completely consumed by the end of the electrolysis reaction. The conduction salt will generally be degradable and will not leave contaminants in the liquid which would require further purification.

The current applied to the cell is pulsed current. Without wishing to be bound by any theory it will be understood that strong acids are normally good electrical conductors. However, hydrofluoric acid is a weak acid i.e. it has a low dissociation constant and therefore it has a very low conductivity at high concentration.

Although some of the prior art processes suggest the use of pulsed currents, the reduction of the arsenic in these prior art processes is inefficient. In this connection, it is noteworthy that arsenic is the slowest metal of all metals in electrochemical deposition. However, in the present invention, the presence of the conduction salt overcomes the drawbacks of the prior art processes as it also acts as an arsenic oxidation inhibitor. Thus it is possible to carry out the process of the present invention efficiently using pulsed currents. The pulsed current may be sinusoidal, square, or triangular, with square being particularly preferred. The current may be symmetric or asymmetric. In a preferred arrangement, a negative asymmetric current is used. Any suitable current may be used. The current frequency may be from O to about 2000 kHz with frequencies in the region of about 200 kHz being particularly preferred. The duty cycle, also known as the cycle ratio, can vary between 0 and 100% but is preferably from about 50 to about

/D /0.

The process of the present invention may be used to treat any liquid containing the arsenic. However, in one example, the liquid comprising the arsenic may be hydrogen fluoride. The hydrogen fluoride may be undiluted or may be present in an aqueous solution at any concentration. Where undiluted hydrogen fluoride is used, the electrolysis will be carried out at a suitable temperature and pressure such that the hydrogen fluoride is in the liquid state.

The process may also be suitable for treatment of fluorosilicic acid, phosphoric acid (including various forms thereof), and other acids, acid mixtures, acidic or neutral effluents containing arsenic.

The process may be carried out as a continuous or batch process.

The temperature at which the electrolysis is carried out will be selected to be the most suitable for the electrolysis being carried out. Suitable temperatures include those between the temperatures of from about -60⁰C and about 100⁰C. More preferred temperatures are those of from about 7°C and to about 14°C.

The temperature of the system may be controlled by any suitable means. In one arrangement the temperature may be controlled by the circulation of a chilling liquid. Where the electrolysis ceil has a double casing, the chilling liquid will generally circulate in the space in the double casing. It will generally also be used to control liquid in the storage vessel and to an off-gas condenser.

The electrochemical process may be carried out in any suitable apparatus. The electrolysis cell may be made of any suitable material provided that it is resistant to the compositions being treated. Suitable materials include plastics such as fluoro polymers for example polyvinylidene fluoride, polytetrafluoroethylene and the like. Alternative suitable materials include metals such as nickel, lead, silver, platinum, stainless steels, nickel alloys, chromium, and combinations thereof.

The cell in which the electrolysis may be carried out may be of any suitable configuration. For example, it may be cylindrical, parallelepiped, spiral or a tube bundle.

The electrodes used in the process for the present invention may be made from any suitable material. Suitable materials include stainless steels, nickel, gold, copper, or carbon. These may be used alone or in combination. Where carbon is to be used it may be in any of its various forms. For example, it may be graphite in its glassy forms, diamond, sapphire and the like, hi one arrangement, the anode may be a carbon electrode and the cathode may be formed from stainless steel . The stainless steel cathode may be protected by a deposit of metallic arsenic. This has the benefit of preventing the contamination of the solution. This is particularly important where the process is being used to remove arsenic from ultra pure hydrogen fluoride for use in the microelectronics industry.

A reference electrode may also be used. The reference electrode may be made from any suitable material. However, it will generally be made from material which is adapted to withstand the corrosive solutions. A suitable material is polytetrafluoroethylene or other fluoropolymers.

The reference electrode will generally have a stable and fixed potential and thus will allow the adjustment of the control of the cathode potential that changes during the electrodeposition with changing ion concentration. The presence of the reference electrode will enable the potential to be kept at all times in the range where As metal is formed and this is as set out in the Pourbaix Diagram. It will be understood that reduction of arsenic to the toxic gas AsHb is not desirable.

The electrodes may be of any suitable configuration. For example, they may be plates or they may be of another configuration including cylindrical, tubular, or spiral. The electrodes, whatever the configuration, may be porous or non-porous. In addition, the electrodes may be formed from spherical particles. The apparatus may include an impulse generator. The generator provides asymmetric impulses of rectangular form allowing the displacement of the duty cycle.

As indicated above, any suitable reference electrode may be used. Classical reference electrodes are made of a ceramic membrane, which may be known as a crucible. This membrane acts as an electrolytic bridge. However, as most ceramic materials contain SiO₂, they are not resistant to hydrofluoric acid. In addition, most materials will be attacked by hydrogen fluoride and contamination by the hydrogen fluoride will occur. To overcome these difficulties, an improved reference electrode has been provided.

Thus, according to a second aspect of the present invention there is provided a reference electrode of a novel design which allows very fine control of the electrochemical cell and overcomes difficulties detailed above.

In a preferred embodiment of the second aspect of the present invention, the reference electrode comprises an electrolyte bridge made of PTFE. The bridge may include a junction membrane produced from NAFION™ or any porous material that is resistant to an acidic environment such as hydrogen fluoride. Suitable resistant materials include sapphire or TiO₂. NAFION is a trade mark of DuPont.

The use of the reference electrode of this second aspect of the present invention in the process of the above-mentioned first aspect of the present invention allows fine control of the electrochemical cell.

The size of the cell and the spacing of the electrodes will be selected by the skilled man to be appropriate for the liquid being treated. However, in general, the distance between the electrodes will be m the region of from about 0.1 mm to about 1000 mm.

The liquid to be treated will generally be pumped through the electrolysis cell. The pump may be a membrane pump. The velocity will generally be selected to be compatible with the arsenic reduction speed of the electrode. Any suitable velocity may be used. Suitable velocities include those in the range of 0.001 to 20m/s. More particular, the velocity may be in the range of 0.1 to The potential of the cathode will be set and controlled to allow the reaction of the arsenic ions to the metallic state. For an acid medium a potential in the range of from about +0.3 to about -0.6 Vz⁷NHE where NHE is the Normal Hydrogen Electrode may be used. For a neutral medium, a potential in the range of from about -0.24 to about -1 V/NHE may be used. A suitable potential for any alkaline medium which might be used can be calculated from the Pourbaix diagram. In any event, the potential should be set to avoid the formation of arsine since this is a very toxic gas-

As indicated above, a reference electrode may be used. The control of the potential will be made using the three electrodes. Measuring of the operating potential will be carried out between the reference electrode and cathode. The adjustment of the potential of the cathode may be carried out by increasing or decreasing the difference in potential ΔU between the cathode and the anode, in this mode the anode plays the role of the auxiliary electrode.

Once the ion potential of the arsenic is reached, the process is then changed to "amperostatic" piloting mode by maintaining the same current as the one measured at the start when the correct reduction potential is reached. First, in batch operation, the electrodeposition is carried out at a constant voltage, which is potentiostatic operation, until the diffusion limit of the medium is reached. This is observed by a reduction in current, In order to overcome the concentration limit, the voltage is then increased and the current kept constant which is the amperostatic operation referred to above.

The process of the present invention may be carried out under an inert gas such as nitrogen in order to avoid humidity condensation.

It will be acknowledged that the process of the present invention can be integrated as part of other production plants. Alternatively, it may be provided as a mobile unit for single or temporary operations. The present invention will now be described, by way of example, with reference to the accompanying drawings in which:

Figure 1 is a schematic representation of a suitable electrochemical cell for the present invention; and

Figure 2 is a reference electrode according to the second aspect of the present invention.

The electrolysis cell 1 in which the electrochemical process of the present invention may be carried out is illustrated in Figure 1. The cell 1 , includes a gold plated plate for the anode and a copper plate for the cathode. The cell 1 is connected by polytetrafluoroethylene tabes to a double- wall electrolyte reservoir 2 where a refrigerant liquid is circulated at a controlled temperature. A reference electrode 3 is immersed in the electrolyte reservoir.

A fluoropolymer membrane pump 4 is circulating in loop the electrolyte to be purified between the cell and the reservoir.

An inert purge gas, such as nitrogen, is blown onto the electrolyte surface in the reservoir. The nitrogen flows through a condenser 5 and an alkaline absorption tower before venting to a suction hood. The cell 1 is placed in a cooling vessel in order to maintain the temperature.

An adjustable pulsed current power generator 6 is connected in a "3 electrode setup". A millivoltmeter 7 is connected between the reference electrode and the cathode and an amperometer 8 is connected in the electrical circuit.

As the electrolysis is carried out, the reduced arsenic precipitates partly in the solution or forms a deposit on the electrode. The arsenic can then be removed by any suitable means. Suitable means include separation techniques such as filtration.

The present process will now be illustrated with reference to the following examples which are not to be considered as limiting. Examples 1 and 2

A series of two runs using alternating current were performed. In each run, the electrolytic cell equipped with gold plated stainless steel anode and a copper cathode was circulated with approximately 400 g of anhydrous hydrogen fluoride at a rate of 1.2 1/min. The surface area of the anode and cathode is 0.5 dm². The electrodes are set so that they are about 2 mm apart.

The respective starting concentration of arsenic impurity was determined for each run. The conduction salt N₂H₄F was added to the solution. The temperature of the cell for the first run was from 7 to 12 ⁰C and for the second run was 9 to 14 ⁰C. The voltage of the alternating electrical current was adjusted using a power generator and the current was allowed to run for the times set out in Table 1. The frequency was set at 200 kHz and the cycle ratio at 75%. The operation mode was amperostatic.

A sample of the liquid was taken and analysed. Data corresponding to the analysis before the electrolytic reduction and the analysis of the liquid after the electrolytic reduction for the respective runs are set out in Table 1.

Table 1

Increasing the time of the run will increase the conversion. In an alternative arrangement, running time can be reduced by using larger surface area electrodes.

Claims

1. A process for reducing arsenic in an acidic liquid comprising arsenic in an oxidized form, wherein the liquid is subjected to electrolysis in a single step and in a single compartment electrolytic cell using pulsed current in the presence of a conduction salt.

2. A process according to Claim 1 wherein the conduction salt is hydrazine fluoride, hydrazine sulphate, hydrazine nitrate, hydrazine hydroxide or hydrogen peroxide.

3. A process according to Claim 2 wherein the conduction salt is hydrazine fluoride.

4. A process according to any one of Claims 1 to 3 wherein the conduction salt is added in an amount such that it is completely consumed by the end of the electrolysis.

5. A process according to any one of Claims 1 to 4 wherein the conduction salt is degraded and does not contaminate the liquid.

6. A process according to Claim 1 wherein the current frequency is from 0 to about 2000 kHz,

7. A process according to Claim 6 wherein the current frequency is about 200 kHz.

8. A process according to any one of Claims 1 to 7 wherein a duty cycle is from 0 and 100%.

9. A process according to Claim 8 wherein the duty cycle is from about 50 to about 75%.

10. A process according to any one of Claims 1 to 9 wherein the liquid containing the arsenic is hydrogen fluoride, fluorosilicic acid, or phosphoric acid.

1 1. A process according to any one of Claims 1 to 10 wherein the electrolysis is carried out at temperatures of from about -6O⁰C to about 100⁰C.

12. A process according to Claim 11 wherein the electrolysis is carried out at temperatures of from about 7°C and to about 14°C.

13. A process according to any one of Claims 1 to 12 wherein a reference electrode is used.

14. A reference electrode comprising an electrolyte bridge made of PTFE.

15. A reference electrode according to Claim 14 wherein the bridge includes a junction membrane produced from NAFION, sapphire or TiO₂.