GB2540745A

GB2540745A - Metal dissolution with nitric acid

Info

Publication number: GB2540745A
Application number: GB1512461.3A
Authority: GB
Inventors: Crossley David
Original assignee: William Blythe Ltd
Current assignee: William Blythe Ltd
Priority date: 2015-07-16
Filing date: 2015-07-16
Publication date: 2017-02-01
Also published as: GB201612349D0; GB2542248A; GB2542248B; WO2017009667A1; GB201512461D0

Abstract

A fast and efficient industrial scale process for producing metal nitrate solutions by metal dissolution using nitric acid which overcomes NO gas treatment requirements by reacting the NO gaseous by-products with oxygen within the reaction vessel. The method is particularly useful for copper, silver, indium and thallium nitrate production. The process comprises provision of the said metal in solid particulate form, provision of nitric acid in aqueous solution, reaction of the metal with the nitric acid in a reaction medium having a headspace containing oxygen, where the metal particles are agitated in the nitric acid by means of an impeller to produce a turbulent admixture of the nitric acid with the headspace containing oxygen, the headspace being maintained above atmospheric pressure, and being replenished with gaseous oxygen to maintain a headspace oxygen concentration of 21% or greater by volume, the temperature being maintained in the range 20 deg C to the boiling point of the nitric acid.

Description

Metal Dissolution with nitric acid

The present invention relates to the dissolution of metals with nitric acid in an industrial process, being a process at a scale wherein the amount of metal being dissolved is upward of 10kg.

Background

Metal nitrate Solutions are commonly prepared by reacting a metal, a metal oxide, a metal hydroxide or a metal carbonate with nitric acid. A particular issue with the reaction of nitric acid with metals that are more electronegative than hydrogen is that, a mixture of NO and N02, commonly referred to as NOx, is formed rather than hydrogen being liberated. Commercially important metals in this category include copper, silver, indium and thallium. When dissolution with nitric acid occurs with a given metal (X) then the reaction scheme is: X(s) + 4HN03(I) = X(N03)2(l) + 2H20(I) + 2N02(g) 3X(s) + 8HN03(I) = 3X( N03)2(l) + 4H20(I) + 2NO(g)

Hence a by-product of the reaction is the aforementioned NOx. Since there are strict statutory limitations to the release of this material into the atmosphere and environmental consequences if it is released then it is necessary to treat the gaseous effluent from the process to remove the NOx.

In conventional production units the NO and N02, that are formed as gases, are absorbed in water in a train of packed column scrubbers to form dilute nitric acid that may be recycled. The absorption of NOx in water is a complex process involving a number of reactions which may be simplistically expressed as: 2NO(g) + 02(g) = 2N02(g) 2N02(g) + H20(l) = HN03(l) + HN02(l) 2HN02(I) = H20(l) + NO(g) + N02(g)

The process of absorption of N02 to form nitric acid leads to the formation of nitrous acid which disproportionates to liberate NO which is sparingly soluble in water and N02 which is soluble and reacts to form further nitric and nitrous acid. The design of the absorption system must therefore allow for the provision of an oxidising agent to continually drive the NO to N02 reaction in either the liquid or gaseous phases. Most commonly this is done by drawing large volumes of atmospheric air through the absorption train - the higher the partial pressure of Oxygen relative to that of NO the faster the oxidation reaction but this leads to large volume absorption towers and a significant residual NOx content in the exit gas stream which is typically abated by absorption in an alkaline solution or reduction to nitrogen. To reduce the volume of the towers, gaseous Oxygen may be introduced into the gas phase or the liquid phase to improve the rate of NO to N02 oxidation and some designers advocate addition of oxidising agents such as Hydrogen Peroxide into the liquid phase - though this does carry it with it the risk of Hydrogen Peroxide decomposition if an accumulation is allowed to build up and then become acidified. The dilute nitric acid recovered by this process may be returned to process if it is of a suitable concentration. However this normally means that it is necessary to concentrate the dilute acid by evaporation to make it usable, unless it is diverted for use in another application.

The absorption of NOx to form nitric acid is an exothermic process and so the liquor circulated over the absorption equipment has to be cooled to remove the heat of reaction otherwise the temperature within the absorption system rises and absorption efficiency is lost.

Relevant prior disclosures of the dissolution of metals with nitric acid include: CN101698498A uses a feed of hydrogen peroxide in addition to the nitric acid that is charged to the reaction vessel. The hydrogen peroxide acts as an oxidant that reacts with the NO and N02 gases. However, this process has a narrow safety margin since the hydroxide may consistently decompose, particularly under acidic conditions.

Particularly relevant is JPS63270418, this discloses the dissolution of copper solid with nitric acid in which a recirculating loop serves to spray the nitric acid over the copper and in doing so enables contact with atmospheric oxygen to enable a proportion of the NOx to be converted into nitric acid. This process, however, has difficulties since recirculation necessitates a pumping of a solution comprising suspended metal solids particles and the atomisation of the recirculating liquid so as to provide contact with atmospheric oxygen. This requires filters and these filters quickly become blocked since the metal dissolution process gives rise to continually finer metal particles. The process is therefore better carried out with large blocks of metal which give a limited surface area and so reduce reaction rate and must be replaced before they disintegrate into smaller particles which will hinder the filtration and atomisation process. A further issue with this process is that a considerable headspace is required for the recirculated acid to contact the atmospheric oxygen so as to provide adequate opportunity for nitric acid to form from the NOx This gives rise to a large reaction vessel and also a requirement for a large volume of atmospheric air which must be contained, particularly at the end of reaction, before being cleaned and released. CN103482591A discloses a copper dissolution at below atmospheric pressure at <0.05MPa (0.5 Bar).

There is therefore a need for an improved metal dissolution process using nitric acid in which the production of NOx effluent is reduced or eliminated, or at least the necessity to treat such potential effluent is reduced or eliminated. There is also a need to provide a means of metal dissolution using nitric acid with improved kinetics. Whilst improved kinetics can be achieved by using a higher reaction temperature, because of the intrinsically exothermic nature of the process there is a practical upper limit at which the heat release would cause the system to boil and so improvement in reaction kinetics must be achieved by measures other than increase in reaction temperature. A further issue regarding metal dissolution using nitric acid is that it is desirable that the end product is a concentrated solution of metal nitrate with limited residual nitric acid content such as may be used directly for the manufacture of metal carbonate or mixed metal carbonate catalyst precursors without need for energetically expensive post-processing steps such as evaporative concentration or from which the metal nitrate may be readily isolated as a solid with minimal need for further processing of the mother liquor to render harmless the residual nitric acid content.

The present invention

The present invention in its various aspects is as set out in the appended claims.

The metals used in metal dissolution to provide metal nitrates of the present invention metals which are more electronegative than hydrogen (i.e. metals with a positive reduction potential as expressed electron volts). These are metals which are oxidized by nitric acid to cause them to go into solution and in so doing reduce the oxidation state of the nitrogen component of nitric acid from 4+ to 2+ thus causing its conversion from nitric to nitrous acid which then disproportionates to liberate NO and N02. A primary benefit of the present invention is that it permits metal nitrate solutions of concentrations close to saturation and with minimal free nitric acid content to be produced in a closed vessel with negligible release of NOx to atmosphere. A secondary benefit of the present invention is that this process will achieve metal dissolution rates at least comparable with those achieved by conventional processes. A third benefit of the present invention is that unlike conventional processes which uses a large excess of the metal and of nitric acid the process may be conducted using close to stoichiometric quantities of the metal and nitric acid to yield solutions with the required metal content without need for evaporation or post production processing to remove or neutralise un-reacted nitric acid. A fourth benefit of the present invention is the prevention by imposition of a high degree of process control, of perturbations in reaction rate, for example, those caused by an accumulation of nitrous acid which would occur if the reaction is run below 50°C and which auto catalyses the reaction between nitric acid and metals and would lead to surges in NOx release which could overwhelm an external abatement system.

Unlike conventional metal - nitric acid reactions this process is conducted in a closed agitated vessel operated under pressure and co fed with nitric acid and Oxygen. Operating an exothermic reaction that produces gas as part of the reaction could lead to pressure build-up and a requirement for venting or overpressure risks. By simultaneously reacting the nitrous acid formed in the liquid phase; by the dissolution of the metal with dissolved Oxygen to form nitric acid; by causing any NO which escapes to the reactor head space to react with Oxygen to form N02; and by promoting reabsorption of N02 from head space into the liquid phase build-up of pressure in the reactor head space is prevented. This is achieved by operating the reaction under a constant overpressure of oxygen maintained by the oxygen supply and by making use of the oxygen supply pressure to both drive kinetically the conversion of NO to N02 and to promote dissolution of N02 in accordance with Le Chatelier's principle.

Within the reactor the following reactions occur where X is the metal which for the purpose of the example is assumed to be divalent: X(s) + 4HN03(I) = X(N03)2(l) + 2H20(I) + 2N02(g) 3X(s) + 8HN03(I) = 3X(N03)2(I) + 4H20(I) + 2NO(g) 2NO(g) + 02(g) = 2N02(g) 2N02(g) + H20(l) = HN03(l) + HN02(l) 2HN02(I) = H20(l) + NO(g) + N02(g)

Achievement of the required reactions of the present invention preferably comprises one or more of the steps: 1. Achieving rapid transport of the products of the reaction - metal nitrate solution - from the metal surface and bringing fresh reactants - nitric acid - to the metal surface. This may be achieved by use of an external circulation loop either circulating liquor through a static bed of metal or spraying liquor onto a static bed but coarse particles of un-dissolved metal could damage the circulation pump and fine particles could blind strainers or spray nozzles in the circulation line. The preferred embodiment of the current invention circumvents this by maintaining the reaction system within a single vessel and using agitation as the method for transporting reaction products from the metal surface and fresh reactants to the metal surface. In the most preferred embodiment of the invention the agitation system is configured so that it is not immersed in the metal bed but induces good off bottom movement/suspension of the metal bed thus avoiding damaging side loads on the agitator shaft which could lead to shaft seal failure. 2. Achieving effective temperature control by application of heat to bring the reactor contents to the preferred start temperature and by then removing heat of reaction so that the reactor contents are held at a temperature below the temperature at which they would boil should pressure control be lost and the working pressure of the vessel fall to atmospheric pressure. This may be achieved by use of an external circulation loop incorporating a heat exchanger but because of the risk of blockage and fouling due to the presence of un-dissolved metal and because of the risk of loss of containment consequent on the mechanical failure of the heat exchanger or circulation pump in the preferred embodiment of the invention this is achieved by use of reaction vessel heating /cooling coils and most preferentially coils on the outside of the reaction vessel and good bulk movement of the reactor contents achieved by the use of blades on the agitation system configured to achieve high liquid flows and hence good mass transfer. 3. Feeding nitric acid to the process at a rate which is less than the rate at which nitric acid will react with the metal so that there is no risk of an accumulation of nitric acid developing with attendant risk of a run-away reaction occurring, and by then maintaining excess oxygen in the process; by using oxygen to ensure that nitrous acid and NO produced by the reaction between the metal and nitric acid are converted to nitric acid as soon as produced. 4. Operating the process at elevated pressure, preferably greater than 0.2 MPa g (2 Bar g) and less than 0.5 MPa g (5 Bar g) to promote dissolution of Oxygen in the liquid phase - in accordance with le Chatelier's Principle and Henry's Law- and thus secure maximum liquid phase conversion of nitrous acid back to nitric acid - and also to maximise the partial pressure of Oxygen in the head space and thus maximise the conversion of NO to N02 and its subsequent re-absorption in the liquid phase - again in accordance with le Chatelier's Principle. 5. Using Oxygen to cause nitrous acid to be converted back to nitric acid and NO to be oxidised to N02 which is then reabsorbed to form nitric acid rather than inorganic oxidants such as Potassium Permanganate which would introduce impurities (manganese salts) to process or Hydrogen Peroxide Solution which is unstable in under acidic conditions and air which would introduce a mass of inert material to process (Nitrogen) which would have to be periodically purged from process to avoid filling the reactor head space with nitrogen thus creating the need for an abatement system to remove NO and N02 from the purge gas. 6. Introducing Oxygen into the liquid phase to secure oxidation of nitrous acid to nitric acid in the liquid phase and thus minimise escape of NO and N02 to the head space. This may be done by promoting absorption of oxygen from the head space into the liquid phase or by the use of an external circulation loop with an eductor to draw oxygen into the reaction system. Preferably the reaction is promoted by the use of sintered metal diffusers which create micro bubbles of Oxygen which dissolve readily in the reaction system. 7. Maximizing the liquor surface at the 'air water' interface in the reactor to maximise rate of adsorption of N02 from the reactor head space plus re-adsorption of Oxygen from the head space. This may be achieved by the use of an external circulation loop and spray nozzle but these suffer the risk of blockage by particles of un-reacted metal and increase the risk of loss of containment of the reactor contents by addition of the circulation pump and external pipe work. In the preferred embodiment of the invention liquor surface area at the 'air/water' interface may be maximised by use the use of hollow shaft impellors configured to draw head space gasses into the reaction system and release them though the eye of an impellor, by the use of an agitation system designed to create a collapsing vortex at the liquor surface or by the use of an agitation system designed to create a mass of droplets at the liquor surface.

Preferably a single agitation system is used to agitate the metal particles, such as an impeller, to promote rapid renewal of the surface film around the metal particles (transport of reaction products from and reagents to metal surface), heat removal and gas re-absorption from the head space, thus eliminating all of the problems cause by use of some kind of external circulation/heat removal loop. The use of the single agitation system to promote very good bulk liquid mixing within the reaction vessel combined with good heat transfer and use of fixed nitric acid addition rate and a constant head space Oxygen over-pressure also leads to homogeneous reaction conditions and thus minimises the risk of perturbations in reaction rate.

More specifically it has been found that an impeller to agitate the metal in the dissolving liquor, i.e. the nitric acid, provides better NO production. This is in contrast to spray on techniques which in principle provide an equally high surface area for dissolution. A further preferred feature is the circulation of head space gas into the nitric acid. A preferred combination therefore includes a stirrer, having a stem with attached impellers. The stem is preferably a hollow stem and arranged so that an upper end of the hollow stem accesses the head space and a lower end is in proximity to an impeller arranged to agitate the metal in the nitric acid that has a corresponding access from the hollow stem. Head space gas may therefore be arranged to circulate from the head space to the impeller and this has been found to greatly accelerate the rate of reaction giving copper dissolution. The impeller is preferably arranged so as to create a reduced pressure in the nitric acid liquid and so draw down gas from the head space trough the hollow stem as part of a stirring action of the impeller. The impeller may comprise paddles having leading and trailing edges and giving rise to turbulence so as to create a reduced pressure at the trailing edge to provide the aforementioned effect of headspace gas recirculation.

Detailed description of the invention

The reactor vessel is charged with the batch stoichiometric requirement of water-this is a function of the desired end concentration of the metal nitrate solution and allows for the water added to process via the heel of metal nitrate solution added in the next stage. A heel being a term of art describing a priming of the reaction vessel with a reacted solution composition (such as usually derived from a previous batch) which appears to 'kick start' the reaction and in particular the dissociation process so as to decrease reaction time. This is particularly effective for copper, silver indium and thallium metals.

The reactor is then preferably charged with a heel of the metal nitrate Solution which may be of the same concentration as the desired end product or at a proportionally greater concentration depending on the quantity of any additional water added. This may range from 1 to 100% by weight of metal batch charge but an amount of 30 to 40% by weight of metal batch charge has been found to be most beneficial in enhancing reaction start up kinetics.

The combination of batch water requirement and the heel of metal nitrate solution are necessary both to ensure that the metal particles can be mobilised by the reaction system and to ensure that the reactor heat transfer surface can be wetted to promote heat transfer to warm the batch at start up and then removal heat of reaction once batch is started

The reactor is then charged with the metal in the form of chopped scrap, flakes or some other divided form which can be mobilised by the reactor agitation system.

The quantity of metal added may either be batch stoichiometric requirement or an excess may be added.

The reactor vessel is then closed up, head space purged with gaseous oxygen (such as to a local exhaust scrubber) to principally remove nitrogen from the head space and then head space pressure increased to a pressure greater than atmospheric - no upper limit on the working pressure has been identified but experience suggests that an optimal comprise between nitrate and oxide production reaction rate for copper, silver, indium and thallium is 1.0 to 5 bar - preferentially 2 bar to 3 bar and most preferably 2.4 to 2.6 bar.

The reactor contents are then raised to a temperature between ambient and boiling point which is sufficient to suppress the formation of nitrous acid which can act as an autocatalyst - the optimum reaction temperature has been found to be in the range 60 to 80°C.

Nitric acid of the concentration necessary to achieve the required product composition - after allowing for water addition - is then metered into the reactor at a fixed rate and Oxygen is added to the reactor to maintain the head space pressure at the fixed value.

The rate of nitric acid addition is determined by the rate at which the reaction between nitric acid and the metal can be sustained within the reactor and is a function of the reactor design -experience suggests that rates as high as 2.0 gm metal dissolved/unit 100% HN03 added/minute are achievable

The oxygen supply rate is regulated to provide the oxygen needed to drive the oxidation of NO to N02, preferably by a pressure controller to maintain a constant pressure in the vessel.

Oxygen may be introduced directly to the reactor head space or it may introduced subsurface via either a sparge pipe or preferentially sintered metal diffusers which generate microbubbles and so promote rapid absorption of oxygen

The rate of reaction is fixed by the rate of nitric acid addition and the reaction is complete when the batch requirement of nitric acid is added.

The reactor contents may then be settled to allow any excess metal to settle to the bottom of the reactor and then the contents of the vessel may be either completely discharged from the vessel or the made quantity may be removed leaving behind the heel which will be used in the subsequent batch.

At batch end the reactor headspace contains substantially oxygen which can be vented to a local effluent abatement system before the reactor is recharged.

In a second embodiment of the invention renewal of the surface layer of the liquor around the metal may be achieved by an external circulation loop rather than by an agitation system - drawing liquor from either above or below the metal bed and circulating though the bed though preferentially the liquor should be circulated up though the bed to promote gas disengagement. This embodiment would permit metal to be used in a form which could not be mobilised by agitation - e.g. bundles of wire or chopped plate.

The volume of the headspace is preferably between 0.5 and 5 times the volume of the nitric acid and no greater than 5 times. The reaction kinetics decreasing considerably above 5 times.

The headspace oxygen concentration may be made as high as 100% by weight, particularly at the end of reaction if operated as a batch process, so as to drive the reaction to completion and avoid a residual headspace requiring significant clean up before release to the atmosphere. To optimise reaction kinetics the initial headspace oxygen concentration may be at or near 100% by weight, particularly when used in conjunction of a heal of metal nitrate solution derived from a previous batch reaction of the process. Thus a cyclical production process is envisaged.

The reaction may be a batch reaction, this enables conversion to be driven to at or near 100% providing a metal nitrate product requiring no or minimal post processing. At or near can be defined as 95 to 100% by weight. The oxygen concentration may be alternatively by weight in all instances as a secondary option.

Surprising it has been found that at the preferred high oxygen concentrations of the present invention little or no copper oxides are formed when the metal to be dissolved is copper. This may be due to the optimised reaction kinetics from the combination of higher pressure, low temperature and metal agitation in the acid, which is employed. This is also found also with thallium and indium; and to a lesser extent with silver, with a negligible effect for Gold.

The headspace may be absent carbon dioxide, which can poison the reaction pathway.

Advantageously the headspace may comprise a noble gas, the noble gas may include helium (which for present purposes if defined as having no weight). This reduces the possibility of hazard at high oxygen levels.

Nitric acid for use in the present invention is preferably of a concentration from 50 to 80%, preferably 60 to 70% as HN03 by weight.

The total nitric acid present in relation to the metal to be dissolved is present from a stoichiometric molar equivalent to a 10% molar excess, preferably in the range of stoichiometric molar equivalent to 2% excess, most preferably at stoichiometric molar equivalent. Further preferably when used at a 2% excess or less when the oxygen concentration is set at 100% at the end of the reaction.

Preferably an excess of metal is used so that metal nitrate end product may be separated from residual metal by filtration the nitric acid level being low or exhausted. The possibility does not benefit from high oxygen concentration as the residual reaction to remove the last nitric acid requires a slow reaction 'tail' raising the possibility of metal oxide creation. MPa g and Bar g (g for gauge) refer to pressure over atmospheric pressure as opposed to MPa or Bar which refer to absolute pressure. Hence, 0.5 Bar would be below atmospheric pressure, 0.5 Bar g would be above atmospheric pressure. For the present invention atmospheric pressure is taken as 0.1 MPa or 1 Bar. If a conflict in units arises the values expressed in Bar prevail.

Claims

Claims,

1. A process for the production of metal nitrates, the process comprising the steps: provision of a metal having a positive electrochemical potential as measured in electron volts, and the metal being in solid particulate form, provision of nitric acid in aqueous solution, reaction of the metal with the nitric acid, the reaction medium having a headspace containing oxygen, characterised in that, the metal particles are agitated in the nitric acid by means of an impeller, and wherein said agitation provides directly or indirectly turbulent admixture of the nitric acid with the headspace containing oxygen, the headspace containing oxygen being maintained at a process pressure, the process pressure being above atmospheric pressure, the headspace containing oxygen being replenished with gaseous oxygen to maintain said process pressure so as to maintain a headspace oxygen concentration of 21% or greater by volume, the temperature of the reaction medium is maintained in the range 20°C to the boiling point of the nitric acid concentration used at said process pressure used,

2. The process of claim one wherein the process pressure is in the range 0.2 to 0.6 MPa (2 to 6 bar).

3. The process of claim 2 were in the process pressure is in the range 0.25 to 0.35 MPa (2.5 to 3.5 bar).

4. The process of any preceding claim wherein the process temperature is maintained in the range 60°C to 80°C.

5. The process of any preceding claim wherein the volume of the headspace is no greater than five times the volume of the nitric acid.

6. The process of any preceding claim where the headspace oxygen concentration is 100% by weight.

7. The process of any preceding claim wherein the gaseous oxygen under pressure is introduced into the nitric acid solution on route to replenishing the headspace.

8. The process of claim 7 wherein the gaseous oxygen is introduced as a dispersion in the form of a micro bubbles of a size less than lOOpm created by the use of a sintered metal diffuser at the inlet of the oxygen.

9. The process of any preceding claim wherein the process is a batch process.

10. The process of claim 9 wherein the total nitric acid present in relation to the metal to be dissolved is present from a stoichiometric molar equivalent to a 10% molar excess.

11. The process of any preceding claim which employs an initial metal charge which may range from the stoichiometric amount of metal which is to be dissolved to produce the desired quantity of the end product to an excess which may be higher than twice, with a preferred range of 10 to 30% stoichiometric excess.

12. The process of any preceding claim wherein the nitric acid is progressively introduced so as to react with the metal until the metal is substantially or completely dissolved.

13. The process of any preceding claim wherein the process is initiated by the provision of a concentrated metal nitrate solution (also termed a heel), preferably derived from a previous instance of the process of the invention, the metal nitrate being the metal nitrate of the metal to be dissolved, the metal nitrate solution being admixed with the metal, subsequent to which the nitric acid is introduced.

14. The process of any preceding claim wherein the impeller used for agitation of the reaction mixture is configured to draw the headspace gas through the liquid, such as by means of an impeller stem having hollow central core.

15. The process of any preceding claim where the impellor used for the agitation of the reaction mixture is configured to increase the liquor surface at the 'air/water' interface in the reactor by either creating a collapsing vortex or by causing extreme turbulence at the liquid surface resulting in droplet formation

16. The process of any preceding claim wherein the metal is selected from copper, silver, indium and thallium.

17. Wherein the metal is copper or silver.