WO2018099243A2

WO2018099243A2 - Nox abatement method for precious metal refinery and recycling processes

Info

Publication number: WO2018099243A2
Application number: PCT/CN2017/109342
Authority: WO
Inventors: Christian Nowottny; Zhenggang YU; Wengang Li
Original assignee: Heraeus Precious Metal Technology China Co Ltd; Heraeus Site Operations GmbH and Co KG
Current assignee: Heraeus Precious Metal Technology China Co Ltd; Heraeus Site Operations GmbH and Co KG
Priority date: 2017-11-03
Filing date: 2017-11-03
Publication date: 2018-06-07
Anticipated expiration: 2020-05-03
Also published as: CN111295238A; WO2018099243A3; WO2019085569A1; CN111295238B

Abstract

The present invention relates to a process for NO_x removal wherein an aqueous liquid, at least one gaseous and/or at least one liquid oxidant, and a NO_x-containing off-gas are fed into a venturi mixing unit, thereby obtaining a turbulent aqueous reaction zone in the venturi mixing unit, and a HNO₃-enriched aqueous liquid leaves the venturi mixing unit.

Description

NOX ABATEMENT METHOD FOR PRECIOUS METAL REFINERY AND RECYCLING PROCESSES

The present invention relates to a process for NO_x removal from a NO_x-containing off-gas which is generated e.g. in a precious metal refinery or recycling process.

Background

Precious metals are used for manufacturing catalysts, electronic devices, space materials, biomedical devices and jewelry. As the resources of precious metals are limited and they are of high economical value, efficient precious metal refinery and recycling processes (such as hydrometallurgical methods) have been developed. A review about hydrometallurgical refinery and recycling processes is provided e.g. by M.K. Jha et al., Hydrometallurgy, 133 (2013) , pp. 23–32.

In precious metal refinery and recycling processes, but also in other chemical processes carried out on an industrial scale, high amounts of poisonous NO_x-containing off-gas might be produced, e.g. by reacting precious metals such as Au or Pt with aqua regia or by decomposing nitric acid, nitrous acid, or salts thereof. To comply with environmental regulations, efficient NO_x abatement technologies are needed. Different from NO_x emissions originating from combustion processes, these industrial chemical processes such as precious metal refinery and recycling processes generate NO_x off-gas in very high concentration.

A beneficial NO_x abatement method should be easy to implement, but nevertheless efficiently remove NO_x from the off-gas even if NO_x concentration is very high. A number of different NO_x abatement methods are known, as described in the review of K. Skalska et al., Science of the Total Environment, 408, 2010, pp. 3976-3989.

NO_x abatement can be achieved by selective reduction of NO_x to nitrogen (N₂) , either in the presence of a catalyst (SCR: “Selective Catalytic Reduction” ) or in the absence of a catalyst (SNCR: “Selective Non-Catalytic Reduction” ) .

In an alternative approach, wet or dry scrubbers can be used for NO_x abatement. In a wet scrubber, a gas stream is brought into contact with the scrubbing liquid. Typically, the scrubbing liquid is an alkaline medium (e.g. due to the presence of sodium hydroxide) , thereby converting the absorbed NO_x to nitrite and nitrate which, however, may cause wastewater disposal problems. Furthermore, wet scrubbers may fail if NO_x is produced at high rates (e.g. well above 50 kg/h) , unless scrubber dimensions are significantly enlarged which, however, is adversely affecting cost efficiency.

WO 2016/180676 A1 describes a process for NO_x abatement wherein a gas mixture of ozone and the NO_x-containing off-gas is provided, thereby oxidizing NO_x to higher nitrogen oxides which are then fed into a wet scrubber and are hydrolyzed to HNO₃ in the aqueous scrubbing liquid. For achieving a high degree of oxidation in this gas-phase reaction, the time period between ozone injection into the NO_x-containing off-gas and entry of said reaction mixture into the wet scrubber needs to be sufficiently long, which in turn means that an appropriate length of the line which connects the ozone injection point to the wet scrubber has to be carefully selected. Alternatively, the ozone might also be directly fed into the wet scrubber and mixed there with the off-gas. As indicated in WO 2016/180676 A1, the oxidation might still work but the water present in the scrubber will slow down the oxidation reaction.

An object of the present invention is to provide a process which is efficiently removing NO_x from a NO_x-containing off-gas, even if NO_x is generated at high rate and high concentration (such as in precious metal refinery or recycling processes) , is cost-efficient and easy to implement, and minimizes the amount of non-recycable materials.

Summary of the Invention

The object is solved by a process for NO_x removal wherein an aqueous liquid, at least one gaseous and/or at least one liquid oxidant, and a NO_x-containing off-gas are fed into a venturi mixing unit, thereby obtaining a turbulent aqueous reaction zone in the venturi mixing unit, and a HNO₃-enriched aqueous liquid leaves the venturi mixing unit.

As known to the skilled person, the venturi effect can be used for mixing gaseous and liquid components. Typically, a venturi mixing unit (i.e. a mixing unit which applies the venturi effect) comprises an inlet portion at which the liquid component is introduced, an outlet portion, and a throat portion (i.e. a portion of lower diameter compared to the diameters of the inlet and outlet portions) which is located between the inlet and outlet portions. As a result of the venturi effect, a gaseous component can be suctioned into the throat portion, thereby obtaining a turbulent mixture of the liquid and gaseous components. The turbulent mixture leaves the venturi mixing unit via the outlet portion.

As indicated above, in precious metal refinery and recycling processes and similar chemical processes carried out on an industrial scale, NO_x off-gas is generated at very high rate and concentration. However, even under these very challenging conditions, it has surprisingly been realized in the present invention that NO_x can be abated very efficiently if the NO_x-containing off-gas and an oxidant are mixed in a venturi mixing unit, thereby obtaining a turbulent aqueous reaction zone in which the NO_x is oxidized to higher nitrogen oxides and these higher nitrogen oxides are instantly hydrolysed to nitric acid (HNO₃) . A HNO₃-enriched aqueous liquid is then leaving the venturi mixing unit and may optionally be subjected to further processing steps (e.g. in a wet scrubber) . So, both the oxidation to higher nitrogen oxides and the hydrolysis of these higher nitrogen oxides to nitric acid are accomplished within said venturi mixing unit. As discussed below in further detail, at least about 90 %of the NO_x off-gas can already be converted to nitric acid in the venturi mixing unit.

The term “HNO₃-enriched aqueous liquid” means that the aqueous liquid which is leaving the venturi mixing unit contains HNO₃ in a somewhat higher concentration than the aqueous liquid fed before into the venturi mixing unit (as a result of oxidizing NO_x to higher nitrogen oxides, followed by hydrolysis to nitric acid) .

Preferably, the gaseous oxidant is oxygen (O₂) or ozone (O₃) or a mixture thereof. The gaseous oxidant might optionally be diluted with an inert carrier gas. If the gaseous oxidant is oxygen, the amount of carrier gas is preferably from 0 vol% (i.e. no carrier gas) to 10 vol%. A preferred liquid oxidant is hydrogen peroxide.

In principle, both a gaseous oxidant and a liquid oxidant can be fed into the venturi mixing unit. However, in a preferred embodiment, either a gaseous oxidant or a liquid oxidant is fed into the venturi mixing unit.

Typically, the venturi mixing unit comprises an inlet portion, an outlet portion, and a throat portion which is located between the inlet and outlet portions, the aqueous liquid enters the venturi mixing unit at the inlet portion, the NO_x-containing off-gas enters the venturi mixing unit at the throat portion (e.g. via a feed line which ends in the throat portion) , and the HNO₃-enriched aqueous liquid leaves the venturi mixing unit at the outlet portion. When the aqueous liquid flows through the throat portion, a reduced pressure is generated and the NO_x-containing off-gas is suctioned into the throat portion. In the presence of an oxidant (which might also be suctioned into the throat portion or might be introduced at the inlet portion) , NO_x is oxidized to higher nitrogen oxides which are then instantly hydrolysed to nitric acid. Due to the venturi effect, a turbulent mixture of the reactants (i.e. a turbulent aqueous reaction zone) is automatically generated in the throat portion and a very high conversion rate of NO_x to HNO₃ is achieved.

The NO_x-containing off-gas can be fed into the throat portion via a feed line which is connected to a NO_x-off-gas-source and ends in the throat portion. Due to the venturi effect, the NO_x-containing off-gas is suctioned into the throat portion.

If a gaseous oxidant is used, it preferably enters the venturi mixing unit at the throat portion, e.g. via a feed line which is connected to a gaseous oxidant source and ends in the throat portion.

Typically, the NO_x-containing off-gas and the gaseous oxidant enter the throat portion via separate feed lines.

If a liquid oxidant is used, it preferably enters the venturi mixing unit at the inlet portion. The liquid oxidant such as hydrogen peroxide and the aqueous liquid may enter the inlet portion of the venturi mixing unit via separate feed lines. Alternatively, the liquid oxidant might be added to the aqueous liquid upstream the venturi mixing unit so that the liquid oxidant and the aqueous liquid enter the venturi mixing unit via the same feed line.

Preferably, the NO_x-containing off-gas is generated in a precious metal (such as Pt, Pd, Rh, Ir, Ru, Ag, Au, or any alloy thereof) refinery or recycling process.

Preferably, the NO_x-containing off-gas is generated by one or more of the following reactions:

- reacting a precious metal or a salt or complex thereof in or with an acid, and/or

- decomposing a nitrate, a nitrite, nitric acid, or nitrous acid.

The acid can be a mineral acid (e.g. a concentrated mineral acid) , such as HNO₃, HCl or H₂SO₄, or a mixture of at least two of these acids. The acid might be a mixture of HNO₃ and HCl, such as aqua regia.

The NO_x-containing off-gas can be generated at a rate of at least 50 kg/hour, more preferably at least 100 kg/hour. Even at these high rates, efficient NO_x abatement is achieved by the process of the present invention.

A part of the HNO₃-enriched aqueous liquid leaving the venturi mixing unit can be recycled, optionally in diluted form, to the inlet portion of the venturi mixing unit.

The HNO₃-enriched aqueous liquid leaving the venturi mixing unit can be fed into a wet scrubber containing an aqueous scrubbing liquid. Any conventional wet scrubber known to the skilled person can be used in combination with the venturi mixing unit.

In an exemplary wet scrubber, an aqueous scrubbing liquid is sprayed in the upper part of the scrubber, moves downwards (thereby optionally passing a packed column or plates) , comes into contact with the HNO₃-enriched aqueous liquid (which was withdrawn from the venturi mixing unit and fed into the wet scrubber) and is collected in the bottom part of the wet scrubber.

If the HNO₃-enriched aqueous liquid from the venturi mixing unit is fed into a wet scrubber, it comes into contact with the aqueous scrubbing liquid, thereby obtaining a HNO₃-enriched aqueous scrubbing liquid (which is typically collected in the bottom part of the wet scrubber) , and a part of said HNO₃-enriched aqueous scrubbing liquid is preferably recycled to the inlet portion of the venturi mixing unit, while another part of said HNO₃-enriched aqueous scrubbing liquid might be recycled to the upper part of the wet scrubber.

The process might be run until a pre-defined HNO₃ concentration value is reached in the HNO₃-enriched aqueous liquid leaving the venturi mixing unit or in the HNO₃-enriched aqueous scrubbing liquid (which is collected in the bottom part of the wet scrubber) .

The process might be conducted in a batch mode or a continuous mode.

At least a part of the HNO₃-enriched aqueous liquid leaving the venturi mixing unit or the HNO₃-enriched aqueous scrubbing liquid might be recycled to the precious metal refinery or recycling process where the NO_x-containing off-gas is generated.

The present invention also relates to the use of a venturi mixing unit for NO_x abatement, preferably in precious metal refinery or recycling. Optionally, the venturi mixing unit can be used in combination with a wet scrubber. Preferably, the wet scrubber is located downstream the venturi mixing unit. With regard to the preferred properties of the venturi mixing unit and the wet scrubber, reference can be made to the statements provided above.

Brief Description of the Drawings

An exemplary embodiment of the present invention is illustrated by way of example in the accompanying drawing:

Figure 1 is a schematic representation of a NO_x abatement system according to the present invention which comprises a venturi mixing unit connected to a wet scrubber, wherein a NO_x-containing off-gas, at least one oxidant and an aqueous medium are introduced into the venturi mixing unit, and the HNO₃-enriched aqueous medium which leaves the venturi mixing medium is fed to the wet scrubber.

Detailed Description of the Invention

Referring to Figure 1, a preferred embodiment of the present invention is discussed below in further detail.

A venturi mixing unit 1 comprises an inlet portion 2, an outlet portion 3, and a throat portion 4 which is located between the inlet portion 2 and the outlet portion 3. The throat portion 4 has a diameter which is lower than the diameters of the inlet 2 and outlet 3 portions. As known to the skilled person, if a liquid flows through the throat portion of a venturi mixing unit, a reduced pressure is generated and a gas can be suctioned into said throat portion (e.g. via a feed line which is connected to a gas source and ends in the throat portion) .

Via feed line 5, the throat portion 4 of the venturi mixing unit 1 is connected to the source where the NO_x-containing off-gas is generated. Preferably, the NO_x-containing off-gas is generated by a precious metal refinery or recycling process. Typically, in this type of process, NO_x-containing off-gas is generated at very high rate, e.g. 50 kg/hour or even higher (such as at least 100 kg/hour) .

Via feed line 7, an aqueous liquid is fed into inlet portion 2 and is forced to flow through throat portion 4. A reduced pressure is generated ( “venturi effect” ) and NO_x-containing off-gas is suctioned into throat portion 4 via feed line 5. An appropriate volume flow rate of the aqueous liquid can be adjusted by the skilled person on the basis of common general knowledge. If the NO_x-containing off-gas is generated e.g. at a rate of at least 50 kg/hour, the volume flow rate of the aqueous liquid which is fed into inlet portion 2 of the venturi mixing unit 1 might be e.g. at least 1000 liters/hour.

For oxidizing NO_x to higher nitrogen oxides, a gaseous oxidant (preferably O₂ or O₃ or a mixture thereof) or a liquid oxidant (preferably hydrogen peroxide) is fed into the venturi mixing unit 1.

In Figure 1, both a feed line 6 for a gaseous oxidant and a feed line 9 for a liquid oxidant are shown. However, in a preferred embodiment, either a gaseous oxidant or a liquid oxidant is fed into the venturi mixing unit.

Feed line 6 is connected to a source of a gaseous oxidant, e.g. a source providing oxygen in a concentration of at least 90 vol%. Again, due to the venturi effect, the gaseous oxidant is suctioned into throat portion 4.

As the NO_x-containing off-gas and the gaseous oxidant are suctioned into throat portion 4, they are intimately mixed with the aqueous liquid flowing through throat portion 4, and a turbulent aqueous reaction zone is obtained.

In this turbulent aqueous reaction zone, NO_x is oxidized to higher nitrogen oxides and these higher nitrogen oxides are then hydrolysed to nitric acid (HNO₃) . Accordingly, a HNO₃-enriched aqueous liquid (if compared to the aqueous liquid which has previously entered the venturi mixing unit 1 via the inlet portion 2) leaves the venturi mixing unit via outlet portion 3.

If a liquid oxidant such as hydrogen peroxide is used, it is preferably fed into the venturi mixing unit 1 via the inlet portion 2. As shown in Figure 1, the liquid oxidant might be added via feed line 9 to the aqueous liquid in feed line 7 upstream the venturi mixing unit so that the liquid oxidant and the aqueous liquid enter the venturi mixing unit 1 via the same feed line (i.e. feed line 7 in Figure 1) . Alternatively, the liquid oxidant and the aqueous liquid may enter the inlet portion 2 of the venturi mixing unit 1 via separate feed lines (i.e. feed line 9 would not end in feed line 7 but end in inlet portion 2) .

In the present invention, it has been realized that NO_x, even if generated at very high rate, can be abated very efficiently if the NO_x-containing off-gas and an oxidant are mixed in a venturi mixing unit, thereby obtaining a turbulent aqueous reaction zone in which the NO_x is oxidized to higher nitrogen oxides and these higher nitrogen oxides are then instantly hydrolysed to nitric acid (HNO₃) . Both oxidation to higher nitrogen oxides and hydrolysis of these higher nitrogen oxides to nitric acid are accomplished within said venturi mixing unit. It has turned out that at least about 90 vol%of the NO_x off-gas can already be converted to nitric acid in the venturi mixing unit.

For some purposes, this very high conversion rate of NO_x to nitric acid of at least 90%just in the venturi mixing unit 1 might already be sufficient.

For further increasing the yield of nitric acid, the HNO₃-enriched aqueous liquid leaving the venturi mixing unit 1 at outlet portion 3 can be fed into a wet scrubber 10 via feed line 8. The outlet portion 3 might be directly connected to the wet scrubber via line 8. Alternatively, the outlet portion 3 might be connected to a storage tank which in turn is connected to the wet scrubber 10. In principle, any conventional wet scrubber known to the skilled person can be used in combination with the venturi mixing unit 1. In an exemplary wet scrubber 10 shown in Figure 1, an aqueous scrubbing liquid is sprayed via spraying means 17 in the upper part of the wet scrubber 1, moves downwards, for example, via a packed column or plates 14, and comes into contact with the HNO₃-enriched aqueous liquid (which was withdrawn from the venturi mixing unit 1 and fed into the wet scrubber10 via line 8) , thereby obtaining a HNO₃-enriched aqueous scrubbing liquid which is collected in the bottom part 11 of the wet scrubber 10.

For illustrative purposes, Figure 1 does not show a true size ratio between the venturi mixing 1 unit and the wet scrubber 10. Relative to the size of the wet scrubber 10, the size of the venturi mixing unit has been enlarged in Figure 1.

A part of the HNO₃-enriched aqueous scrubbing liquid is withdrawn (either continuously or batchwise) from the bottom part 11 of the wet scrubber 1 via line 12 and recycled to the inlet portion 2 of the venturi mixing unit via line 7, and a new cycle starts. In this new cycle, the aqueous liquid fed into the inlet portion 2 already contains some nitric acid which was generated in the previous cycle. Again, by reacting NO_x and the oxidant and hydrolysis of the oxidized species to nitric acid, concentration of nitric acid increases and a HNO₃-enriched aqueous liquid leaves the venturi mixing unit 1 via outlet portion 3 and is fed into wet scrubber 10. From cycle to cycle, the concentration of HNO₃ in the HNO₃-enriched aqueous liquid leaving the outlet portion 3 and in the HNO₃-enriched aqueous scrubbing liquid collected in the bottom part of the wet scrubber 10 increase (if compared to the liquid at the same locations in the previous cycle) .

A part of the HNO₃-enriched aqueous scrubbing liquid which was withdrawn via line 12 is preferably recycled to the upper part of the wet scrubber 10 via line 13.

The process might be run until a pre-defined HNO₃ concentration value is reached in the HNO₃-enriched aqueous liquid leaving the venturi mixing unit or in the HNO₃-enriched aqueous scrubbing liquid collected in the bottom part 11 of the wet scrubber. If said pre-defined HNO₃ concentration value is reached, the HNO₃-containing liquid can be withdrawn from the NO_x abatement system via line 16. Optionally, at least a part of said HNO₃-containing liquid withdrawn via line 16 might be recycled to the precious metal refinery or recycling process where the NO_x-containing off-gas is generated.

If there remain low amounts of NO_x, they can be withdrawn from the wet scrubber via line 15. However, in the process of the present invention, at least 90 vol%of the NO_x are already converted to HNO₃ in the venturi mixing unit, and the remaining amounts of NO_x are more or less completely converted to HNO₃ in the wet scrubber.

The NO_x abatement system used in the present invention may additionally contain pumps, valves, cooling units or similar means for process control. These means are not shown in Figure 1. However, based on common general knowledge, the skilled person knows how to implement these means into the NO_x abatement system, if needed.

Claims

A process for NO_x removal wherein an aqueous liquid, at least one gaseous and/or at least one liquid oxidant, and a NO_x-containing off-gas are fed into a venturi mixing unit (1) , thereby obtaining a turbulent aqueous reaction zone in the venturi mixing unit (1) , and a HNO₃-enriched aqueous liquid leaves the venturi mixing unit.
The process according to claim 1, wherein the gaseous oxidant is oxygen or ozone or a mixture thereof; and/or wherein the liquid oxidant is hydrogen peroxide.
The process according to claim 1 or 2, wherein the venturi mixing unit comprises an inlet portion (2) , an outlet portion (3) , and a throat portion (4) which is located between the inlet (2) and outlet (3) portions, the aqueous liquid enters the venturi mixing unit (1) at the inlet portion (2) , the NO_x-containing off-gas enters the venturi mixing unit (1) at the throat portion (4) , and the HNO₃-enriched aqueous liquid leaves the venturi mixing unit (1) at the outlet portion (4) .
The process according to one of the preceding claims, wherein the gaseous oxidant enters the venturi mixing unit (1) at the throat portion (4) ; and/or wherein the liquid oxidant enters the venturi mixing unit (1) at the inlet portion (2) .
The process according to one of the preceding claims, wherein the NO_x-containing off-gas is generated in a precious metal refinery or recycling process.
The process according to one of the preceding claims, wherein the NO_x-containing off-gas is generated by one or more of the following reactions:

- reacting a precious metal or a salt or complex thereof in or with an acid,

- decomposing a nitrate, a nitrite, nitric acid, or nitrous acid.
The process according to one of the preceding claims, wherein the NO_x-containing off-gas is generated at a rate of at least 50 kg/hour.
The process according to one of the preceding claims, wherein a part of the HNO₃-enriched aqueous liquid leaving the venturi mixing unit (1) is recycled, optionally in diluted form, to the inlet portion (2) of the venturi mixing unit (1) .
The process according to one of the preceding claims, wherein the HNO₃-enriched aqueous liquid leaving the venturi mixing unit (1) is fed into a wet scrubber (10) containing an aqueous scrubbing liquid.
The process according to claim 9, wherein the HNO₃-enriched aqueous liquid from the venturi mixing unit comes into contact with the aqueous scrubbing liquid, thereby obtaining a HNO₃-enriched aqueous scrubbing liquid, and a part of said HNO₃-enriched aqueous scrubbing liquid is recycled to the inlet portion (2) of the venturi mixing unit (1) .
The process according to one of the preceding claims, wherein the process is run until a pre-defined HNO₃ concentration value is reached in the HNO₃-enriched aqueous liquid leaving the venturi mixing unit (1) or the HNO₃-enriched aqueous scrubbing liquid.
The process according to one of the preceding claims, wherein the process is conducted in a batch mode or a continuous mode.
The process according to one of the preceding claims, wherein at least a part of the HNO₃-enriched aqueous liquid leaving the venturi mixing unit (1) or at least a part of the HNO₃-enriched aqueous scrubbing liquid is recycled to the precious metal refinery or recycling process where the NO_x-containing off-gas is generated.
Use of a venturi mixing unit (1) , optionally in combination with a wet scrubber (10) , for NO_x abatement.