WO2019085569A1

WO2019085569A1 - NOx ABATEMENT METHOD FOR PRECIOUS METAL REFINERY AND RECYCLING PROCESSES

Info

Publication number: WO2019085569A1
Application number: PCT/CN2018/098923
Authority: WO
Inventors: Christian Nowottny; Zhenggang YU; Wengang Li
Original assignee: Heraeus Precious Metal Technology (China) Co., Ltd.; Heraeus Site Operations Gmbh & Co. Kg
Priority date: 2017-11-03
Filing date: 2018-08-06
Publication date: 2019-05-09
Also published as: WO2018099243A2; CN111295238B; WO2018099243A3; CN111295238A

Abstract

Process for NOx removal wherein an aqueous liquid, oxygen and/or ozone, and a NOx-containing off-gas generated in a precious metal refinery or recycling process 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 (1).

Description

NO x 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 in a precious metal refinery or recycling process.

The term “NO _x “used herein is known to the skilled person as collective name for nitrogen oxides with nitrogen having an oxidation state in the range of +2 to +4.

Background

Precious metals are used for manufacturing catalysts, electronic devices, space materials, bio-medical 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 hy-drometallurgical 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 containing off-gas in very high NO _x 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 (N ₂) , 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 prob-lems. 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 af-fecting 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 HNO ₃ in the aqueous scrubbing liquid. For achieving a high degree of oxidation in this gas-phase reaction, the time period be-tween 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.

US2013/0224093 A1 discloses a process for integrating treatment of flue gas through NO _x removal, SO _x removal and mercury removal, which could avoid high energy consumption resulting from high temperature required for the reaction, and a corresponding apparatus thereof. The process comprises the splitting of the off gas to a sub-stream off gas flow line as a carrier for the oxidizing agent. In the sub-stream line, a heating chamber is set for the decomposition of the oxidizing agent to form free radicals having stronger oxidative capacity than the original oxidants used, these free radicals carried in the sub-stream off gas are then combined with the unheated mainstream off gas, and react with the reducible contents such as NO _x, SO _x and mercury vapor in the off gas to generate NO _x, SO _x and mercury ions with higher oxidation state, wherein all the acidic products and mercury ions will be removed in the subsequent gas scrubbing device.

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 consumption of chemicals and the formation of non-recyclable materials.

Summary of the Invention

The object is solved by a process for NO _x removal wherein an aqueous liquid, a gaseous oxi-dant, and a NO _x-containing off-gas are fed into a venturi mixing unit, thereby obtaining a turbu- lent aqueous reaction zone in the venturi mixing unit, and a HNO ₃-enriched aqueous liquid leaves the venturi mixing unit, wherein the gaseous oxidant is oxygen and/or ozone, wherein the NO _x-containing off-gas is not a flue gas, and wherein the NO _x-containing off-gas is gener-ated in a precious metal refinery or recycling process.

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-containing off-gas is generated at very high rate and concentration. However, even under these very challenging conditions, it has surpris-ingly been realized in the present invention that NO _x can be abated very efficiently if the NO _x-containing off-gas and a gaseous 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 (HNO ₃) . A HNO ₃-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 of the NO _x-containing off-gas can already be converted to nitric acid in the venturi mixing unit.

The term “HNO ₃-enriched aqueous liquid” means that the aqueous liquid which is leaving the venturi mixing unit contains HNO ₃ 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) .

The gaseous oxidant is oxygen (O ₂) or ozone (O ₃) 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%.

In an optional embodiment, hydrogen peroxide might be fed into the venturi mixing unit as addi-tional liquid oxidant.

Typically, the venturi mixing unit comprises an inlet portion, an outlet portion, and a throat por-tion 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 HNO ₃-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 the gaseous oxidant (which might also be suctioned into the throat portion or might be introduced at the inlet portion) , NO _x is oxi-dized to higher nitrogen oxides which are then instantly hydrolysed to nitric acid. Due to the ven-turi effect, a turbulent mixture of the reactants (i.e. a turbulent aqueous reaction zone) is auto-matically generated in the throat portion and a very high conversion rate of NO _x to HNO ₃ is achieved. The temperature in the venturi mixing unit may be in the range of from 15 to 60 ℃ or, preferably, 20 to 30 ℃ during said mixing.

The NO _x-containing off-gas can be fed into the throat portion via a feed line which is connected to a NO _x-containing 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.

The gaseous oxidant 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 sepa-rate feed lines.

If hydrogen peroxide is optionally used as additional liquid oxidant, it preferably enters the ven-turi mixing unit at the inlet portion. The hydrogen peroxide and the aqueous liquid may then en-ter the inlet portion of the venturi mixing unit via separate feed lines. Alternatively, the hydrogen peroxide might be added to the aqueous liquid upstream the venturi mixing unit so that the hy-drogen peroxide and the aqueous liquid enter the venturi mixing unit via the same feed line.

The NO _x-containing off-gas is not a flue gas (not a combustion off-gas) . Its NO _x content may be in the range of, for example, 30 to 100 vol%, in particular in the range of 50 to 100 vol%. The NO _x-containing off-gas is typically free of mercury (Hg) and free of sulfur oxides (SO _x) .

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 gen-erated 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 HNO ₃, HCl or H ₂SO ₄, or a mixture of at least two of these acids. The acid might be a mixture of HNO ₃ and HCl, such as aqua regia.

Needless to say, a person skilled in the art will understand that not only a part or substream of the NO _x-containing off-gas is fed into the venturi mixing unit. Rather, the entire NO _x-containing off-gas is fed into the venturi mixing unit in order to achieve the desired removal of NO _x.

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 HNO ₃-enriched aqueous liquid leaving the venturi mixing unit can be recycled, op-tionally in diluted form, to the inlet portion of the venturi mixing unit.

The HNO ₃-enriched aqueous liquid leaving the venturi mixing unit can be fed into a wet scrub-ber containing an aqueous scrubbing liquid. The simplest example of an aqueous scrubbing liquid to start with is water. 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 op-tionally passing a packed column or plates) , comes into contact with the HNO ₃-enriched aque-ous 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 HNO ₃-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 HNO ₃-enriched aqueous scrubbing liquid (which is typically collected in the bottom part of the wet scrubber) , and a part of said HNO ₃-enriched aqueous scrubbing liquid is preferably recycled to the inlet portion of the venturi mixing unit, while another part of said HNO ₃-enriched aqueous scrubbing liquid might be recycled to the upper part of the wet scrubber.

The process might be run until a pre-defined HNO ₃ concentration value is reached in the HNO ₃-enriched aqueous liquid leaving the venturi mixing unit or in the HNO ₃-enriched aqueous scrub-bing 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 HNO ₃-enriched aqueous liquid leaving the venturi mixing unit or the HNO ₃-enriched aqueous scrubbing liquid might be recycled to the precious metal refinery or recycling process where the NO _x-containing off-gas is generated.

Brief Description of the Drawings

An exemplary embodiment of the present invention is illustrated by way of example in the ac-companying 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, a gaseous oxidant and an aqueous medium are introduced into the venturi mixing unit, and the HNO ₃-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. 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 suc-tioned 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 (O ₂ or O ₃ or a mixture thereof) is fed into the venturi mixing unit 1. Optionally, hydrogen peroxide may be fed into the venturi mix-ing unit 1 as additional liquid oxidant.

In Figure 1, both a feed line 6 for the gaseous oxidant and a feed line 9 for the optional hydro-gen peroxide liquid oxidant are shown.

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 suc-tioned 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 (HNO ₃) . Accordingly, a HNO ₃-enriched aqueous liquid (if compared to the aqueous liquid which has previously entered the venturi mix-ing unit 1 via the inlet portion 2) leaves the venturi mixing unit via outlet portion 3.

If hydrogen peroxide is used as additional liquid oxidant, it is preferably fed into the venturi mix-ing unit 1 via the inlet portion 2. As shown in Figure 1, such optional 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 optional 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 optional 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 the gaseous 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 hydro-lysed to nitric acid (HNO ₃) . 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%of the NO _x of the NO _x-containing off-gas can already be con-verted 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 HNO ₃-enriched aqueous liquid leaving the ven-turi 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 por-tion 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 combina-tion with the venturi mixing unit 1. In an exemplary wet scrubber 10 shown in Figure 1, an aque-ous 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 HNO ₃-enriched aqueous liquid (which was withdrawn from the venturi mixing unit 1 and fed into the wet scrubber10 via line 8) , thereby obtaining a HNO ₃-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 HNO ₃-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 gaseous oxidant and hydrolysis of the oxidized species to nitric acid, concentration of nitric acid increases and a HNO ₃-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 HNO ₃ in the HNO ₃-enriched aqueous liquid leaving the outlet portion 3 and in the HNO ₃-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 pre-vious cycle) .

A part of the HNO ₃-enriched aqueous scrubbing liquid which was withdrawn via line 12 is pref-erably recycled to the upper part of the wet scrubber 10 via line 13.

The process might be run until a pre-defined HNO ₃ concentration value is reached in the HNO ₃-enriched aqueous liquid leaving the venturi mixing unit or in the HNO ₃-enriched aqueous scrub-bing liquid collected in the bottom part 11 of the wet scrubber. If said pre-defined HNO ₃ concen-tration value is reached, the HNO ₃-containing liquid can be withdrawn from the NO _x abatement system via line 16. Optionally, at least a part of said HNO ₃-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 %of the NO _x are already con-verted to HNO ₃ in the venturi mixing unit, and the remaining amounts of NO _x are more or less completely converted to HNO ₃ 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, a gaseous 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 HNO ₃-enriched aqueous liquid leaves the ven-turi mixing unit, wherein the gaseous oxidant is oxygen and/or ozone, wherein the NO _x-containing off-gas is not a flue gas, and wherein the NO _x-containing off-gas is generated in a precious metal refinery or recycling process.
The process according to claim 1, wherein the NO _x-content of the NO _x-containing off-gas lies in the range of 30 to 100 vol%.
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 HNO ₃-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 en-ters the venturi mixing unit (1) at the throat portion (4) .
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 HNO ₃-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 HNO ₃-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 8, wherein the HNO ₃-enriched aqueous liquid from the venturi mixing unit comes into contact with the aqueous scrubbing liquid, thereby obtaining a HNO ₃-enriched aqueous scrubbing liquid, and a part of said HNO ₃-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 HNO ₃ concentration value is reached in the HNO ₃-enriched aqueous liquid leaving the venturi mixing unit (1) or the HNO ₃-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 HNO ₃-enriched aqueous liquid leaving the venturi mixing unit (1) or at least a part of the HNO ₃-enriched aqueous scrubbing liquid is recycled to the precious metal refinery or recycling process where the NO _x-containing off-gas is generated.