There are many applications that require packed bed absorbers for scrubbing SO2. Some applications previously blogged about include refinery sulfur recovery unit tail gas treatment units (SRU/TGTU), geothermal power generation, and waste oil re-refining to name a few. Another SO2 scrubber example is gas cleaning equipment for a mineral processing application to recover gold and silver from mine sites with low grade ore. Discovered and patented in the early 1900 by Charles Merrill and later refined by Thomas Crowe, the Merrill-Crowe process is a common separation technique for removing gold and silver from a cyanide solution using zinc dust. Gold and silver precipitate (concentrate) is the product of the Merrill-Crowe process. Furnaces are then used to recover ingots from smelting Merrill-Crowe precipitate. Fluxes are mixed with the precipitate to bring impurities to the surface. The precious metals settles down and can then be easily removed.
Envitech is currently building an SO2 scrubber to treat the exhaust gases from a melting furnace for a South American precious metals refinery mining project. Furnace off-gases are in the 300oF to 500oF range and contain particulate and SO2. A bag-house removes particulate from the gas before passing downstream to the packed bed absorber for SO2 removal. Gas is first cooled to saturation in an evaporative quencher using re-circulated water. In the case of a pump failure, an emergency spray nozzle provides an independent source of water controlled by a thermocouple. Water that has not evaporated flows from the quencher into the packed bed absorber sump. Gas from the quencher enters the bottom of the absorber and travels vertically upward through a packed bed. Recirculated water is sprayed over the packed bed and mixes with the counter current gas. A dilute solution of plant-supplied sodium hydroxide is metered into the scrubber recirculation line to neutralize acid gases and is controlled by pH of the absorber sump liquid. The scrubbing water is collected in the sump and is re-circulated to the top of the packed bed and to the quencher. A blowdown stream is taken from the recirculation line to purge the system of reaction products. After the packed bed, the gas passes through an entrainment separator to remove water droplets entrained in the gas during scrubbing. The gas then exits the scrubber and is exhausted to atmosphere through an ID fan and stack.
The scrubber scope of supply includes a quencher, packed bed absorber, instruments, control system, recirculation pump, piping, valves, and fittings, ID fan, and metering pump. The unit will be pre-assembled to the fullest extent possible with pre-mounted instruments pre-wired to a control panel. The assembly will be broken down as necessary for shipment and packaged for export to South America. Bilingual submittals are provided for the O&M manuals and engineering submittals. The scrubber will be delivered and started up in Q1 of 2019.
Click on the link below for a case study on the Merrill-Crowe Refining Furnace SO2 Scrubber and a packed bed absorber cut sheet.