Removal of NOx and SOx emissions from gaseous effluents

Abstract: A process for the removal of NO x and SO x , from a gaseous effluent comprising: a) passing the gaseous effluent through a first aqueous alkaline scrubber to remove higher oxidation forms of NO x and SO x , and acid gases; b) treating the gaseous effluent from step a) with a gas mixture containing ozone to oxidise NO x , and SO x , to higher oxidation; and c) passing the resultant gas mixture through a second aqueous alkaline scrubber to remove additional higher oxidation forms of NO x , and SO x , formed in step b).

Description: This invention relates to the removal of noxious compounds NO x and SO x from gaseous effluents, particularly those from metal pickling operations. 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 NO x and possibly SO x . 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 NO x , SO x and acid gases before it can be vented to the atmosphere. NO x refers to the nitrogen oxides commonly present in waste gas effluents primarily NO, N 2 O 3 , NO 2 and N 2 O 5 . 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 NO x formed exists as NO, the effluent gas from a pickling operation commonly contains a much higher percentage of higher nitrogen oxides, particularly NO 2 . As a general comparison, combustion typically produces at least about 90 vol. percent NO with the balance being NO 2 whereas the effluent gas from a pickling operation typically contains only from about 60 to 65 vol. percent NO with the balance NO 2 . These considerations apply to SO x formation as well. NO x and SO x 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 NO x and SO x in an effluent gas stream as described above. However, more efficient systems were required to meet ever-tightening NO x 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 NO 2 . The second scrubber contacts NO 2 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 ClO 2 . United States patent US-A-3957949 describes the removal of NO x 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 SO x by a conventional desulphurisation process prior to the removal of NO x because the sodium chlorite would be reduced by the SO x in the waste gas. European patent application EP-A-0199037 describes the separation of NO x from flue gases wherein the NO content is oxidised to NO 2 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 NO 2 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 NO x removal is selective catalytic reduction (SCR) in which catalysts reduce NO x to nitrogen gas. The problem with the use of such systems to treat pickling line effluents is that the HF and H 2 SO 4 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 NO x 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 O 3 for each mole of NO x 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 processing operation. It would therefore be a significant advance if a process could be provided which would efficiently remove NO x and SO x from gaseous effluents for use in metal pickling operations. The invention is primarily directed to an improvement in the removal of NO x and SO x 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, HNO 3 , H 2 SO 4 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 FeCl 2 , Fe(NO 3 ) 2 and the like, and hydrogen. If the H 2 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 NO x and SO x 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 NO x and SO x from a gaseous effluent taken from a pickling bath comprising: a) passing the gaseous effluent from the pickling bath through a first aqueous alkaline scrubber to remove higher oxidation forms of NO x and SO x , and acid gases; b) treating the gaseous effluent from step a) with a gas mixture containing ozone to oxidise lower oxidation forms of NO x to higher oxidation forms; and c) passing the resultant gas mixture through a second aqueous alkaline scrubber to remove additional higher oxidation forms of NO x formed in step b), characterised in that the gaseous effluent from said step a) also contains lower oxidation forms of SO x and the lower oxidation forms of SO x are oxidised to higher oxidation forms in said step b), the higher oxidation forms of SO x 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 NO x and SO x present to higher oxidation forms. 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 NO x and SO x than found in combustion gases. For these reasons, although there is a considerable breadth of technology concerning NO x and SO x 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 NO x 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 NO x and SO x 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 NO x present in the effluent. This means, for example, that NO 2 as well as any N 2 O 3 and N 2 O 5 present will be converted to HNO 3 or NaNO 3 . 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. NaNO 3 or CaSO 4 , can be concentrated and discarded as solid/liquid conventional waste. The alkaline solution also neutralises any acid, i.e. HF and HNO 3 , 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 NO x and SO x primarily in their lower oxidation state, i.e. NO and SO 2 . 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 NO x 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 NO x 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 NO x and SO x 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 NO x 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.%, NO x and SO x 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 NO x and SO x 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 NO x plus SO x present in the effluent. This represents an improvement in efficiency over previously known processes. 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 H 2 + O 3 → H 2 O + O 2 . 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 NO x and SO x 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 NO x and SO x 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.

Claims: A process for the removal of NO x and SO x from a gaseous effluent taken from a pickling bath comprising: a) passing the gaseous effluent from the pickling bath through a first aqueous alkaline scrubber to remove higher oxidation forms of NO x and SO x , and acid gases; b) treating the gaseous effluent from step a) with a gas mixture containing ozone to oxidise lower oxidation forms of NO x to higher oxidation forms; , and c) passing the resultant gas mixture through a second aqueous alkaline scrubber to remove additional higher oxidation forms of NO x formed in step b), characterised in that the gaseous effluent from said step a) also contains lower oxidation forms of SO x and the lower oxidation forms of SO x are oxidised to higher oxidation forms in said step b), the higher oxidation forms of SO x 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 NO x and SO x present to higher oxidation forms. A process according to claim 1, in which the alkaline scrubbers contain sodium hydroxide or calcium hydroxide. A process according to claim 1 or claim 2, in which the concentration of ozone is such that the molar ratio of ozone to NO x plus SO x in the effluent in step b) is from 0.7:1 to 1.4:1. A process according to claim 3, in which the molar ratio of ozone to NO x plus SO x in the effluent in step b) is from 0.9:1 to 1.2:1. A process according to any preceding claim, additionally including the step of determining the concentration of NO x and SO x in the effluent in step a), and adjusting the molar concentration of ozone in step b) in accordance with such determination.

Inventors: ANDERSON MARK H, HWANG SHUEN-CHENG, SAXENA NEERAJ, WORKOSKY ROBERT FRANCIS, ANDERSON, MARK, H., HWANG, SHUEN-CHENG, SAXENA, NEERAJ, WORKOSKY, ROBERT FRANCIS
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 1999
Main classification: B01D53
IPC codes: B01D53