Wallstein Holding GmbH & Co. KG

NOx Scrubber

Nitrous fumes are oxides of nitrogen, the most commonly known being NO und NO2. But also dimers N2O3 and N2O4 occur in significant quantities and must be taken into account when designing a scrubber.

The engineers at Wallstein have many years‘ experience of 

engineering/ designing wet scrubbers for the treatment of nitrous fumes.

If the concentration of NOx is very high, physical prescrubbingsystems come into operation in order to recover nitric acid and reduce the volume of sewage. 

Nitrous fumes are subsequently separated until the required emission limit is reached, preferably using a countercurrent packed-bed column. For this, lye and hydrogen peroxide are added to the washing fluid. 

Examples of use

1) Semiconductor and photovoltaic industries
In the semiconductor and photovoltaic industries, silicon wafers are etched in a mixture of nitric-  and hydrofluoricacids. This process produces nitrous fumes which are removed using our time-proven NOx scrubbers. 

The gas is initially freed from hydrofluoric acid in the presrubbing stage, after which the separation takes place in single- or multi-stage NOx washing column under controlled addition of lye and oxidizing agent.

Two-stage NOx packed-bed scrubber with a prescrubber (not visible in this photo)
Two-stage NOx packed-bed scrubber with a prescrubber (not visible in this photo)

2) Precious metal recycling
In order to separate precious metals, they are dissolved using strong acids, such as nitric acid and aqua regia. Some of the emissions from these processes have an extremely high NOx content. In such cases, a large portion of NO2 is dissolved in water in the first absorption stage (using a combined jet/packed-bed scrubber), and then reused.

Afterwards, the NOx is separated in the packed-bed scrubber using lye and oxidizing agent until its content is reduced below the required emission limit. 

The photo below depicts a NOx scrubber in which the waste gas flows through the scrubber in four stages (twice concurrently and twice countercurrently to the scrubbing agent). The scrubber was assembled and welded on site from prefabricated components in order to utilize the available space to the fullest capacity. The photo shows the pump pressure line of the scrubber with strainer and flow measurement, as well as a collector pipe above for distribution of the scrubbing agent at the scrubber head.

Wallstein supplied the complete exhaust air cleaning system comprising scrubbers, pipelines, instruments and a control unit.

NOx scrubber at a precious metal refinery
NOx scrubber at a precious metal refinery