Cyanide monitoring and treatment under dynamic process conditions (DynCyanide) : final report

During raw iron production in the blast furnace cyanide occurs, partly entering the gas washing water via top gas and scrubber. Project aims were determination of dynamic correlations of BF operations - HCN in the top gas - cyanide in gas washing water for development of a reliable cyanide treatment and comparable cyanide lab and online measurement. Gas washing waters showing max. 4 mg/l CN,free, decreased about 73% in tank after scrubber or in following treatment units (biological, formaldehyde dosage with aeration) up to 99%. Treatment costs consist mainly of pumping energy and chemicals. A Europe-wide tendency to decrease discharge limits is noticeable. An internal reuse of treated gas washing water is not possible without pretreatment. HCN top gas measurement showed a strong variation up to factor 10 with 250 mg/Nm3 maximum. A parametric study showed a maximum correlation of 0.4 (100% correlating = value 1) between the carbonate or total alkalinity and the total cyanide content. Cyanide measurements for cyanide contents < 1 μg/L (LOQ of 0.86 μg/L) were achieved as well an optimized cyanide measurement (deviation: <0.02 mg/L) including a sample pre-treatment. A simulation model describing the CN degradation with O3 was developed. A newly developed catalyst for cyanide treatment was tested in field trials as well as the ozone treatment and different cyanide online analysers (Skalar, Methrom) achieving final cyanide contents < 0.1 mg/L and a reliable cyanide online measurement. Based on this, a guideline for choosing a cyanide treatment method was developed and an economical evaluation performed.