zone 10 and the likely presence of some dissolved ferrous sulfate the solution leaving zone 10 will contain some dissolved no it is undesirable for this solution containing some dissolved no to be brought into contact with the h 2 s containing gas the gas would strip some of this no from the solution thereby contaminating it to prevent this the solution leaving zone 10 through its bottom liquid outlet flows through pipe 19 into a top liquid inlet of gas liquid contactor 15 gas liquid contactor 15 should be at a pressure somewhat below atmospheric the lower its operating pressure the better since the function of gas liquid contactor 15 is to enable no to be stripped from the ferric sulfate solution formed in zone 10 generally contactor 15 will be maintained at a pressure some 5 psig below atmospheric air is drawn into a lower gas inlet of contactor 15 through pipe 21 as the air travels upward it counter currently contacts the ferric sulfate solution moving downward the operation is controlled so that the ferric sulfate solution leaving the bottom outlet of contactor 15 is substantially free of no the stripped no mixed with the incoming air leaves contactor 15 through its upper.