This invention relates to the removal of noxious compounds NOx
and SOx from gaseous effluents, particularly those from metal pickling
Pickling refers to the treatment of metal during processing such as
shaping or size reduction, to remove scale and other metal oxides that have accumulated
during storage or earlier processing. The treatment consists of exposing the metal
to an aqueous bath of strong inorganic acids, typically nitric acid mixed with one
or more of sulphuric acid, hydrofluoric acid or hydrochloric acid. As a result of
the chemical reactions taking place in the bath, and at times due to thermal decomposition
of the acid fumes therein, the atmosphere over the bath contains a high concentration
of NOx and possibly SOx. The atmosphere in the pickling tanks
is typically regularly purged using a mixture of dry air and nitrogen. This exhaust
gas must be treated to significantly reduce NOx, SOx and acid
gases before it can be vented to the atmosphere.
NOx refers to the nitrogen oxides commonly present in waste
gas effluents primarily NO, N2O3, NO2 and N2O5.
These oxides, which have increasing degrees of oxidation in the order given, likewise
have varying degrees of solubility/reactivity with aqueous solutions, preferably
caustic solutions having a pH between 7 and 14. In general, the higher the oxidation
state, the greater the solubility/reactivity with aqueous caustic solutions. Unlike
combustion in which a large percentage of the NOx formed exists as NO,
the effluent gas from a pickling operation commonly contains a much higher percentage
of higher nitrogen oxides, particularly NO2. As a general comparison,
combustion typically produces at least about 90 vol. percent NO with the balance
being NO2 whereas the effluent gas from a pickling operation typically
contains only from about 60 to 65 vol. percent NO with the balance NO2.
These considerations apply to SOx formation as well.
NOx and SOx are currently removed from pickling
lines primary by alkaline scrubbing. The use of multiple scrubbings will remove
about 40% to 50% by volume of the NOx and SOx in an effluent
gas stream as described above. However, more efficient systems were required to
meet ever-tightening NOx emission standards. One such system is a wet
oxidation system which utilises hydrogen peroxide or sodium oxychloride in the first
scrubber to convert NO to NO2. The second scrubber contacts NO2
with a reducing agent, such as sodium hyposulphide, to convert it to nitrogen which
can be vented to the atmosphere. This system suffers from high cost and the possibility
of release of dangerous emissions, such ClO2.
United States patent US-A-3957949 describes the removal of NOx
from a waste gas wherein NO contained in the gas is oxidised by chlorine dioxide
or ozone and then the oxidised gas is brought into contact with aqueous sodium chlorite
in an absorption column. It removes SOx by a conventional desulphurisation
process prior to the removal of NOx because the sodium chlorite would
be reduced by the SOx in the waste gas.
European patent application EP-A-0199037 describes the separation
of NOx from flue gases wherein the NO content is oxidised to NO2
by ozone or another oxidising agent and the gas is then subjected to a washing process
with substances which have a reducing effect, for example calcium sulphite or magnesium
sulphite, which reduce the NO2 to nitrogen and are themselves oxidised
to the corresponding sulphates.
Japanese patent application JP53011164A describes the desulphurising
and denitrifying of waste combustion gas by first contacting the waste gas in a
desulphurising tower with aqueous sodium hydroxide and low concentration aqueous
sodium sulphite obtained from a denitrifying tower. The waste gas is reacted with
ozone to oxidise the NO content. The so-oxidised waste gas is contacted in a denitrifying
tower with high concentration aqueous sodium sulphite.
Another technology utilised to address the problem of NOx
removal is selective catalytic reduction (SCR) in which catalysts reduce NOx
to nitrogen gas. The problem with the use of such systems to treat pickling line
effluents is that the HF and H2SO4 fumes must be initially
removed, such as in a scrubber. Failure effectively to remove such acid fumes will
result in the catalyst becoming plugged or inactivated by the acids. This will result
in the discharge, or "slip", of hazardous substances from the system into the environment.
SCR and other non-catalytic reduction technologies may also require heating the
gas to an optimum temperature for optimal reduction reaction.
Still another technology for the removal of NOx is referred
to as low temperature oxidation. This is, in effect, heat removal followed by an
ozone-based oxidation system followed by a wet scrubber. The molar ratios utilised
for a system are a minimum 1.5 moles of O3 for each mole of NOx
to be removed. In addition to a fairly high rate of ozone consumption, this system
requires a series of process steps, such as the heat exchange, which are not required
for treatment of the low temperature, saturated emissions from a pickling of metals
It would therefore be a significant advance if a process could be
provided which would efficiently remove NOx and SOx from gaseous
effluents for use in metal pickling operations.
The invention is primarily directed to an improvement in the removal
of NOx and SOx from the gaseous effluent of a metal pickling
process in particular. In a pickling process, metal having a scale of metal oxides
is contacted with pickling acids - such as HCl, HNO3, H2SO4
and HF. The acid is transported to the metal surface through the outer layers of
metal oxide, where it reacts to form metal salts - such as FeCl2, Fe(NO3)2
and the like, and hydrogen. If the H2 gas phase layer on the metal surface
is thick, the pickling process is slowed down due to the increased resistance to
acid transport to the metal surface. Hence, any means of lowering the gas phase
resistance would speed-up the pickling rate, increasing the metal throughput.
The invention is generally directed to a process of removing NOx
and SOx from a gaseous effluent from a metal pickling operation comprising
passing the gaseous effluent from the bath through an aqueous alkaline scrubber,
treating the effluent therefrom with ozone to oxidise NO present to higher oxidation
levels and again passing the effluent through an aqueous alkaline scrubber.
In accordance with the invention, there is provided a process for
the removal of NOx and SOx from a gaseous effluent taken from
a pickling bath comprising:
characterised in that the gaseous effluent from said step a) also contains lower
oxidation forms of SOx and the lower oxidation forms of SOx
are oxidised to higher oxidation forms in said step b), the higher oxidation forms
of SOx being removed in said step c), and in that the pH in both aqueous
alkaline scrubbers is from 10 to 14, and in that additional gas mixture containing
ozone is bubbled through the pickling bath to increase the pickling efficiency and
to oxidise a portion of the NOx and SOx present to higher
- a) passing the gaseous effluent from the pickling bath through a first aqueous
alkaline scrubber to remove higher oxidation forms of NOx and SOx,
and acid gases;
- b) treating the gaseous effluent from step a) with a gas mixture containing
ozone to oxidise lower oxidation forms of NOx to higher oxidation forms;
- c) passing the resultant gas mixture through a second aqueous alkaline scrubber
to remove additional higher oxidation forms of NOx formed in step b),
Because of the nature of the pickling process, the gaseous effluent,
which is formed by sweeping the pickling tank with nitrogen or nitrogen-rich gas,
preferably a mixture of nitrogen and air, is not at a high temperature as would
be the case with combustion gases. The gaseous effluent contains some volatilised
strong mineral acids and their decomposition products, and contains a significantly
higher concentration of NOx and SOx than found in combustion
gases. For these reasons, although there is a considerable breadth of technology
concerning NOx and SOx removal, there has heretofore not been
a process which provides for the efficient, i.e. greater than 80% by volume, removal
thereof from pickling process gas effluent.
The first step in the process of the invention is to pass the gaseous
effluent from the pickling bath through an aqueous alkaline scrubber. The pH of
the scrubber should generally be between 7 and 14, but is preferably very basic,
i.e. between pH 10 and 14. It is important to note that it is not necessary to prepare
the gaseous effluent in any way prior to admitting it to the scrubber, such as by
passing it through a heat exchanger, since it is typically not at a significantly
elevated temperature in comparison to typical combustion off-gases. A typical pickling
gaseous effluent will contain from about 1,000 to 10,000 ppm NOx by volume.
The scrubber is contained in a suitable corrosion-resistant container. The effluent
is generally admitted to the bottom of the scrubber and allowed to flow upward through
and react with the aqueous scrubber solution.
The particular configuration of the scrubber is not critical to the
invention so long as it affords a means of causing sufficient contact between the
aqueous solution and the effluent to have a good percentage of the NOx
and SOx present dissolve into the solution. For instance, a vertical
counter current packed or tray column, or a horizontal aqueous spray chamber can
be used. In general, a residence time between about 2 and 20 seconds in the scrubber
is adequate contact for the scrubber to remove a good portion of the impurities
present therein. The aqueous scrubbers as described here must generally be equipped
with means to withdraw a portion of the scrubber solution periodically and either
replace it with fresh solution or clean it of impurities and return it to the scrubber.
In the scrubber, the scrubbing solution reacts primarily with the
higher forms of NOx present in the effluent. This means, for example,
that NO2 as well as any N2O3 and N2O5
present will be converted to HNO3 or NaNO3. Acid products
formed can be removed to a conventional acid waste treatment plant or recycled to
the pickling bath. The aqueous alkaline scrubber may contain sodium hydroxide or
calcium hydroxide, for example, and the salt products formed, e.g. NaNO3
or CaSO4, can be concentrated and discarded as solid/liquid conventional
waste. The alkaline solution also neutralises any acid, i.e. HF and HNO3,
that may have been carried out of the pickling tank. This is obviously important
since it can be very detrimental for such free acids to pass beyond the initial
stage of treatment.
The gaseous effluent from the initial scrubber now contains NOx
and SOx primarily in their lower oxidation state, i.e. NO and SO2.
The effluent is passed to a conventional ozone treatment apparatus that can take
any configuration. In general, the apparatus can comprise a passage, for example
a conduit, to the subsequent step of sufficient length to permit thorough mixing
of gases and reaction between the effluent and ozone so that the low oxidation state
compounds are oxidised to a higher oxidation state.
The ozone is generated by passing oxygen or air through a conventional
generator and can be introduced into the ozone treatment zone at or soon after the
point where the effluent withdrawn from the scrubber is introduced into the treatment
zone. In general, it is preferred to equip the ozone treatment zone with a conventional
analyser device that measures the NOx content of the effluent exiting
the scrubber and adjusts the ozone content in the incoming ozone mixture, i.e. by
controlling the production of ozone, or its flow rate as necessary to provide the
correct concentration for the NOx content in the effluent. A decided
advantage of the process of the invention is that, by combining the initial scrubber
step with the ozone treatment and the subsequent scrubber step to be described,
the amount of active reactant in all steps, i.e. the aqueous scrubber solution and
ozone, respectively, is reduced in comparison to conventional usage.
The gaseous effluent from the ozone treatment is passed to a second
aqueous scrubber, also preferably an aqueous alkaline solution. As above, the water
dissolves the higher forms of NOx and SOx to form corresponding
acids which can be recycled to an acid storage or treatment facility. Alternatively,
the alkaline reacts therewith to form the corresponding neutralised salts which,
as above, can be transported to a conventional waste treatment plant. Since the
treatment in the ozone apparatus oxidises lower forms of NOx to higher
oxidation states and the second scrubber removes the higher oxidation states efficiently,
the effluent from the second scrubber contains less than about 20 vol.%, preferably
less than 10 vol.%, NOx and SOx present in the original effluent.
The effluent can then be emitted as any conventional smoke stack discharge.
The process provided in accordance with the invention is effective
in removing at least 80% by volume of the NOx and SOx present
in the metal pickling effluent utilising only from 0.7 to 1.4, preferably from 0.9
to 1.2, moles of ozone per mole of NOx plus SOx present in
the effluent. This represents an improvement in efficiency over previously known
As an additional step in the improved process of the invention, some
of the ozone produced from the ozone generator is also used to agitate the acid
bath. The ozone introduced can be supplemental to the air used already for this
purpose, or to replace it. The ozone gas flow will provide mechanical agitation
action, while the ozone present will react with the hydrogen bubbles formed during
the de-scaling reaction to form water by the reaction
H2 + O3 → H2O + O2.
The gas bubbles provide acid agitation for enhanced pickling and de-scaling
reaction by removing the hydrogen bubbles away from the metal surface and increasing
the effective area over which the metal is contacted with acid. Excess ozone also
reacts with the NOx and SOx fumes formed in the pickling process
to form higher order, and more soluble, nitrogen and sulphur oxides, thus enhancing
the efficiency of the first aqueous scrubber.
The process of the invention can be utilised to purify the gaseous
effluent stream from pickling processes with most metals, particularly the ferrous
metals, and with other processes involving the use of concentrated inorganic acids,
such as nitric acid, sulphuric acid and the like. In addition to the high efficiency
of removal of NOx and SOx provided by the process, it is advantageous
in that it removes toxic gas phase emissions and converts them to conventional weak
aqueous acids and salt solutions. The weak acids can be used in the acid plant already
present at the pickling facility. The waste salts formed can be discharged to the
salt solution treatment facility, which is also a conventional facility already
present at the pickling plant. By the use of the first scrubber and the ozone treatment,
the total oxidant consumption of the system is lowered compared to other recognised
processes. Finally, since the amount of ozone utilised is carefully controlled to
remain with the defined limit, the process of the invention does not suffer from
having ozone slip into the outlet gas stream.