Rinsing water recovery in the steel industry — a combined UF/NF treatment

In this paper, a two-step membrane treatment for acidic rinsing water is described including project idea, laboratory tests, pilot plant experiments and finally plant construction. The task is to recover metal and sulphuric acid from rinsing water originating from the electro-zinc coating line at Salzgitter Flachstahl. The developed membrane process produces a concentrated metal solution reused as electrolyte in the coating process and an acidic metal-free permeate reused as rinsing water. In a laboratory membrane screening, a suitable ceramic UF membrane could be identified for particle separation. Metal recovery can be obtained with a NF membrane. A metal rejection of 99.2% (Zn) and 99.8% (Fe) has been found in the laboratory tests while the rejection of sulphuric acid is below 30%, allowing acid passage into the permeate as intended. Pilot experiments at the industrial line have been performed to demonstrate technical feasibility. Permeate fluxes between 30 and 70 l/m2h can be realised in the NF step. The concentrate contains more than 20 g/l Zn. With respect to the successful pilot plant operation Salzgitter Flachstahl decided to install the membrane process for rinsing water treatment. Plant operation started in 2005. The payback period on invest has been only 13 months and the plant fulfils all requirements concerning permeate and concentrate quality. The plant consists of pre-filtration, UF with three units (14 m2 each) and NF with three serial units (53 m2 + two times 106 m2).